Steel material suitable for use in sour environment

ABSTRACT

The steel material according to the present disclosure contains a chemical composition consisting of, in mass %, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, P: 0.030% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.10 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, N: 0.0100% or less and O: 0.0100% or less, with the balance being Fe and impurities. The steel material contains an amount of dissolved C within a range of 0.010 to 0.050 mass %. The steel material also has a yield strength within a range of 655 to less than 862 MPa, and a yield ratio of the steel material is 85% or more.

This is a National Phase Application filed under 35 U.S.C. § 371, ofInternational Application No. PCT/JP2019/012304, filed Mar. 25, 2019,the contents of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a steel material, and more particularlyrelates to a steel material suitable for use in a sour environment.

BACKGROUND ART

Due to the deepening of oil wells and gas wells (hereunder, oil wellsand gas wells are collectively referred to as “oil wells”), there is ademand to enhance the strength of oil-well steel materials representedby oil-well steel pipes. Specifically, for example, 80 ksi grade (yieldstrength is 80 to less than 95 ksi, that is, 552 to less than 655 MPa)and 95 ksi grade (yield strength is 95 to less than 110 ksi, that is,655 to less than 758 MPa) oil-well steel pipes are being widelyutilized, and recently requests are also starting to be made for 110 ksigrade (yield strength is 110 to less than 125 ksi, that is, 758 to lessthan 862 MPa) oil-well steel pipes.

Most deep wells are in a sour environment containing corrosive hydrogensulfide. In the present description, the term “sour environment” meansan acidified environment containing hydrogen sulfide. Note that, in somecases a sour environment may also contain carbon dioxide. Oil-well steelpipes for use in such sour environments are required to have not onlyhigh strength, but to also have sulfide stress cracking resistance(hereunder, referred to as “SSC resistance”).

Technology for enhancing the SSC resistance of steel materials astypified by oil-well steel pipes is disclosed in Japanese PatentApplication Publication No. 62-253720 (Patent Literature 1), JapanesePatent Application Publication No. 59-232220 (Patent Literature 2)Japanese Patent Application Publication No. 6-322478 (Patent Literature3), Japanese Patent Application Publication No. 8-311551 (PatentLiterature 4), Japanese Patent Application Publication No. 2000-256783(Patent Literature 5), Japanese Patent Application Publication No.2000-297344 (Patent Literature 6), Japanese Patent ApplicationPublication No. 2005-350754 (Patent Literature 7), National Publicationof International Patent Application No. 2012-519238 (Patent Literature8) and Japanese Patent Application Publication No. 2012-26030 (PatentLiterature 9).

Patent Literature 1 proposes a method for improving the SSC resistanceof steel for oil wells by reducing impurities such as Mn and P. PatentLiterature 2 proposes a method for improving the SSC resistance of steelby performing quenching twice to refine the grains.

Patent Literature 3 proposes a method for improving the SSC resistanceof a 125 ksi grade steel material by refining the steel microstructureby a heat treatment using induction heating. Patent Literature 4proposes a method for improving the SSC resistance of steel pipes of 110to 140 ksi grade by enhancing the hardenability of the steel byutilizing a direct quenching process and also increasing the temperingtemperature.

Patent Literature 5 and Patent Literature 6 each propose a method forimproving the SSC resistance of a steel for low-alloy oil countrytubular goods of 110 to 140 ksi grade by controlling the shapes ofcarbides. Patent Literature 7 proposes a method for improving the SSCresistance of steel material of 125 ksi grade or higher by controllingthe dislocation density and the hydrogen diffusion coefficient todesired values. Patent Literature 8 proposes a method for improving theSSC resistance of steel of 125 ksi grade by subjecting a low-alloy steelcontaining 0.3 to 0.5% of C to quenching multiple times. PatentLiterature 9 proposes a method for controlling the shapes or number ofcarbides by employing a tempering process composed of a two-stage heattreatment. More specifically, in Patent Literature 9, a method isproposed that enhances the SSC resistance of 125 ksi grade steel bysuppressing the number density of large M₃C particles or M₂C particles.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Publication No.62-253720

Patent Literature 2: Japanese Patent Application Publication No.59-232220

Patent Literature 3: Japanese Patent Application Publication No.6-322478

Patent Literature 4: Japanese Patent Application Publication No.8-311551

Patent Literature 5: Japanese Patent Application Publication No.2000-256783

Patent Literature 6: Japanese Patent Application Publication No.2000-297344

Patent Literature 7: Japanese Patent Application Publication No.2005-350754

Patent Literature 8: National Publication of International PatentApplication No. 2012-519238

Patent Literature 9: Japanese Patent Application Publication No.2012-26030

SUMMARY OF INVENTION Technical Problem

As described above, accompanying the increasing severity of oil wellenvironments in recent years, there is a demand for oil-well steel pipesthat are more excellent in SSC resistance than the conventional oil-wellsteel pipes. Therefore, steel materials (for example, oil-well steelpipes) having a yield strength of 95 ksi grade or 110 ksi grade (655 toless than 862 MPa) and excellent SSC resistance may be obtained by usingtechniques other than the techniques disclosed in the aforementionedPatent Literatures 1 to 9.

An objective of the present disclosure is to provide a steel materialthat has a yield strength within a range of 655 to less than 862 MPa (95to less than 125 ksi; 95 ksi grade or 110 ksi grade) and that also hasexcellent SSC resistance.

Solution to Problem

A steel material according to the present disclosure contains a chemicalcomposition consisting of, in mass %, C: 0.20 to 0.50%, Si: 0.05 to0.50%, Mn: 0.05 to 1.00%, P: 0.030% or less, S: 0.0100% or less, Al:0.005 to 0.100%, Cr: 0.10 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to0.050%, N: 0.0100% or less, O: 0.0100% or less, B: 0 to 0.0050%, V: 0 to0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%.Ni: 0 to 0.50% and Cu: 0 to 0.50%, with the balance being Fe andimpurities. The steel material according to the present disclosure alsocontains an amount of dissolved C within a range of 0.010 to 0.050 mass%. The steel material according to the present disclosure also has ayield strength within a range of 655 to less than 862 MPa, and a yieldratio of the steel material is 85% or more.

Advantageous Effects of Invention

The steel material according to the present disclosure has a yieldstrength within a range of 655 to less than 862 MPa (95 ksi grade or 110ksi grade), and also has excellent SSC resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1A is a view illustrating the relation between the amount ofdissolved C and a fracture toughness value K_(ISSC) for respective testnumbers having a yield strength of 95 ksi grade in the examples.

FIG. 1B is a view illustrating the relation between the amount ofdissolved C and a fracture toughness value K_(ISSC) for respective testnumbers having a yield strength of 110 ksi grade in the examples.

FIG. 2A shows a side view and a cross-sectional view of a DCB testspecimen that is used in a DCB test in the present embodiment.

FIG. 2B is a perspective view of a wedge that is used in the DCB test inthe present embodiment.

DESCRIPTION OF EMBODIMENTS

The present inventors conducted investigations and studies regarding amethod for obtaining both a yield strength in a range of 655 to lessthan 862 MPa (95 ksi grade or 110 ksi grade) and excellent SSCresistance in a steel material that it is assumed will be used in a sourenvironment, and obtained the following findings.

If the dislocation density in the steel material is increased, the yieldstrength of the steel material will increase. On the other hand, thereis a possibility that dislocations will occlude hydrogen. Therefore, ifthe dislocation density of the steel material increases, there is apossibility that the amount of hydrogen that the steel material occludeswill also increase. If the hydrogen concentration in the steel materialincreases as a result of increasing the dislocation density, even ifhigh strength is obtained, the SSC resistance of the steel material willdecrease. Accordingly, at first glance it seems that, in order to obtainboth a yield strength of 95 ksi or more and SSC resistance, utilizingthe dislocation density to enhance the strength is not preferable.

However, the present inventors discovered that by adjusting the amountof dissolved C in a steel material, excellent SSC resistance can also beobtained while at the same time adjusting the yield strength to 95 ksior more by utilizing the dislocation density. Although the reason is notcertain, it is considered that the reason may be as follows.

Dislocations include mobile dislocations and sessile dislocations, andit is considered that dissolved C in a steel material immobilizes mobiledislocations to thereby form sessile dislocations. When mobiledislocations are immobilized by dissolved C, the disappearance ofdislocations can be inhibited, and thus a decrease in the dislocationdensity can be suppressed. In this case, the yield strength of the steelmaterial can be maintained.

In addition, it is considered that the sessile dislocations that areformed by dissolved C reduce the amount of hydrogen that is occluded inthe steel material more than mobile dislocations. Therefore, it isconsidered that by increasing the density of sessile dislocations thatare formed by dissolved C, the amount of hydrogen that is occluded inthe steel material is reduced. As a result, the SSC resistance of thesteel material can be increased. It is considered that because of thismechanism, excellent SSC resistance is obtained even when the steelmaterial has the yield strength of 95 ksi or more.

As described above, the present inventors considered that byappropriately adjusting the amount of dissolved C in a steel material,the SSC resistance of the steel material can be increased whilemaintaining a yield strength of 95 ksi or more. Therefore, using a steelmaterial containing chemical composition consisting of, in mass %, C:0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%, P: 0.030% or less,S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.10 to 1.50%, Mo: 0.25 to1.50%, Ti: 0.002 to 0.050%, N: 0.0100% or less, O: 0.0100% or less, B: 0to 0.0050%, V: 0 to 0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to0.50%, W: 0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to 0.50%, with thebalance being Fe and impurities, the present inventors investigated therelation between the amount of dissolved C, the yield strength, and afracture toughness value K_(ISSC) that is an index of SSC resistance.

[Relation Between Amount of Dissolved C and SSC Resistance]

The present inventors first conducted detailed studies regarding therelation between the amount of dissolved C and SSC resistance of a steelmaterial having a yield strength of 95 ksi grade (655 to less than 758MPa). Specifically, with reference to the figures, the relation betweenthe amount of dissolved C and SSC resistance of the steel materialcontaining aforementioned chemical composition and a yield strength of95 ksi grade is described.

FIG. 1A is a view illustrating the relation between the amount ofdissolved C and a fracture toughness value K_(ISSC) for respective testnumbers having a yield strength of 95 ksi grade in the examples. FIG. 1Awas obtained by the following method. FIG. 1A was created using theamount of dissolved C (mass %) and the fracture toughness value K_(ISSC)(MPa Nm) obtained with respect to steel materials for which, among thesteel materials of the examples that are described later, conditions ofthe aforementioned chemical composition is satisfied and a yieldstrength is 95 ksi grade (655 to less than 758 MPa) and a yield ratio is85% or more.

Note that, adjustment of the yield strength of the steel materialsindicated in FIG. 1A was performed by adjusting the temperingtemperature. Further, with respect to the SSC resistance, if thefracture toughness value K_(ISSC) obtained by a DCB test that isdescribed later was 42.0 MPa√m or more, it was determined that the SSCresistance was good.

Referring to FIG. 1A, in a steel material in which the conditions of theaforementioned chemical composition are satisfied, when the amount ofdissolved C was 0.010 mass % or more, the fracture toughness valueK_(ISSC) became 42.0 MPa√m or more, indicating excellent SSC resistance.On the other hand, in a steel material in which the conditions of theaforementioned chemical composition are satisfied, when the amount ofdissolved C was more than 0.050 mass %/9, the fracture toughness valueK_(ISSC) was less than 42.0 MPa√m. In other words, it was clarified thatwhen the amount of dissolved C is too high, conversely, the SSCresistance decreases.

The reason the SSC resistance decreases when the amount of dissolved Cis too high as described above has not been clarified. However, withrespect to the range of the chemical composition and yield strength (95ksi grade) of the present embodiment, excellent SSC resistance can beobtained if the amount of dissolved C is made 0.050 mass % or less.

The present inventors further conducted detailed studies regarding therelation between the amount of dissolved C and SSC resistance of a steelmaterial having a yield strength of 110 ksi grade (758 to less than 862MPa). Specifically, with reference to the figures, the relation betweenthe amount of dissolved C and SSC resistance of the steel materialcontaining aforementioned chemical composition and a yield strength of110 ksi grade is described.

FIG. 1B is a view illustrating the relation between the amount ofdissolved C and a fracture toughness value K_(ISSC) for respective testnumbers having a yield strength of 110 ksi grade in the examples. FIG.1B was obtained by the following method. FIG. 1B was created using theamount of dissolved C (mass %) and the fracture toughness value K_(ISSC)(MPa √m) obtained with respect to steel materials for which, among thesteel materials of the examples that are described later, conditions ofthe aforementioned chemical composition is satisfied and a yieldstrength is 110 ksi grade (758 to less than 862 MPa) and a yield ratiois 85% or more.

Note that, adjustment of the yield strength of the steel materialsindicated in FIG. 1B was performed by adjusting the temperingtemperature. Further, with respect to the SSC resistance, if thefracture toughness value K_(ISSC) obtained by a DCB test that isdescribed later was 27.5 MPa√m or more, it was determined that the SSCresistance was good.

Referring to FIG. 1B, in a steel material in which the conditions of theaforementioned chemical composition are satisfied, when the amount ofdissolved C was 0.010 mass % or more, the fracture toughness valueK_(ISSC) became 27.5 MPa√m or more, indicating excellent SSC resistance.On the other hand, in a steel material in which the conditions of theaforementioned chemical composition are satisfied, when the amount ofdissolved C was more than 0.050 mass %, the fracture toughness valueK_(ISSC) was less than 27.5 MPa√m. In other words, it was clarified thatwhen the amount of dissolved C is too high, conversely, the SSCresistance decreases.

The reason the SSC resistance decreases when the amount of dissolved Cis too high as described above has not been clarified. However, withrespect to the range of the chemical composition and yield strength (110ksi grade) of the present embodiment, excellent SSC resistance can beobtained if the amount of dissolved C is made 0.050 mass % or less.

Therefore, in a case where a steel material contains the aforementionedchemical composition, even if the yield strength is 95 ksi grade (655 toless than 758 MPa) or 110 ksi grade (758 to less than 862 MPa), when theamount of dissolved C is 0.010 to 0.050 mass %, excellent SSC resistancecan be obtained. Accordingly, in the present embodiment, the amount ofdissolved C of the steel material is set within the range of 0.010 to0.050 mass %.

Note that, the microstructure of the steel material according to thepresent embodiment is made a microstructure that is principally composedof tempered martensite and tempered bainite. The term “principallycomposed of tempered martensite and tempered bainite” means that thevolume ratio of tempered martensite and tempered bainite is 90% or more.When the microstructure of the steel material is principally composed oftempered martensite and tempered bainite, in the steel materialaccording to the present embodiment, the yield strength is in a range of655 to less than 862 MPa (95 ksi grade or 110 ksi grade), and a yieldratio (ratio of the yield strength to the tensile strength; in otherwords, yield ratio (YR)=yield strength (YS)/tensile strength (TS)) is85% or more.

A steel material according to the present embodiment that was completedbased on the above findings contains a chemical composition consistingof in mass %, C: 0.20 to 0.50%, Si: 0.05 to 0.50%, Mn: 0.05 to 1.00%0,P: 0.030% or less, S: 0.0100% or less, Al: 0.005 to 0.100%, Cr: 0.10 to1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to 0.050%, N: 0.0100% or less,0:0.0100% or less, B: 0 to 0.0050%, V: 0 to 0.30%, Nb: 0 to 0.100%, Ca:0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to 0.0100%, rare earth metal: 0 to0.0100%, Co: 0 to 0.50%, W: 0 to 0.50%, Ni: 0 to 0.50% and Cu: 0 to0.50%, with the balance being Fe and impurities. The steel materialaccording to the present embodiment contains an amount of dissolved Cwithin a range of 0.010 to 0.050 mass %. In the steel material accordingto the present embodiment, the yield strength is within a range of 655to less than 862 MPa, and the yield ratio is 85% or more.

In the present description, although not particularly limited, the steelmaterial is, for example, a steel pipe or a steel plate. Preferably, thesteel material is an oil-well steel material that is used for oil wells,further preferably is an oil-well steel pipe. In the presentdescription, as described above, the term “oil wells” is generic name ofoil wells and gas wells.

The aforementioned chemical composition may contain B in an amount of0.0001 to 0.0050%.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of V: 0.01 to 0.30% and Nb:0.002 to 0.100%.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of Ca: 0.0001 to 0.0100%, Mg:0.0001 to 0.0100%, Zr: 0.0001 to 0.0100% and rare earth metal: 0.0001 to0.0100%.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of Co: 0.02 to 0.50% and W:0.02 to 0.50%.

The aforementioned chemical composition may contain one or more types ofelement selected from the group consisting of Ni: 0.02 to 0.50% and Cu:0.02 to 0.50%.

The aforementioned steel material may be an oil-well steel pipe.

In the present description, the oil-well steel pipe may be a steel pipethat is used for a line pipe or may be a steel pipe used for oil countrytubular goods (OCTG). The oil-well steel pipe may be a seamless steelpipe, or may be a welded steel pipe. The oil country tubular goods are,for example, steel pipes that are used as casing pipes or tubing pipes.

The aforementioned steel material may be a seamless steel pipe.

If the oil-well steel pipe according to the present embodiment is aseamless steel pipe, even if the wall thickness is 15 mm or more, theoil-well steel pipe will have a yield strength within a range of 655 toless than 862 MPa (95 ksi grade or 110 ksi grade) and will also haveexcellent SSC resistance.

The term “amount of dissolved C” mentioned above means the differencebetween the amount of C (mass %) in carbides in the steel material andthe C content of the chemical composition of the steel material. Theamount of C in carbides in the steel material is determined by Formula(1) to Formula (5) using an Fe concentration <Fe>a, a Cr concentration<Cr>a, an Mn concentration <Mn>a, an Mo concentration <Mo>a, a Vconcentration <V>a and an Nb concentration <Nb>a in carbides (cementiteand MC-type carbides) obtained as residue when extraction residueanalysis is performed on the steel material, and an Fe concentration<Fe>b, a Cr concentration <Cr>b, an Mn concentration <Mn>b and an Moconcentration <Mo>b in cementite obtained by performing point analysisby energy dispersive X-ray spectrometry (hereunder, referred to as“EDS”) with respect to cementite identified by performing a transmissionelectron microscope (hereunder, referred to as “TEM”) observation of areplica film obtained by an extraction replica method.<Mo>c=(<Fe>a+<Cr>a+<Mn>a)×<Mo>b/(<Fe>b+<Cr>b+<Mn>b)  (1)<Mo>d=<Mo>a−<Mo>c  (2)<C>a=(<Fe>a/55.85+<Cr>a/52+<Mn>a/53.94+<Mo>c/95.9)/3×12  (3)<C>b=(<V>a/50.94+<Mo>d/95.9+<Nb>a/92.9)×12  (4)(amount of dissolved C)=<C>−(<C>a+<C>b)  (5)

Note that, in the present description, the term “cementite” meanscarbides containing an Fe content of 50 mass % or more.

Hereunder, the steel material according to the present embodiment isdescribed in detail. The symbol “%” in relation to an element means“mass percent” unless specifically stated otherwise.

[Chemical Composition]

The steel material according to the present embodiment is suitable forusing in the sour environment. The chemical composition of the steelmaterial according to the present embodiment contains the followingelements.

C: 0.20 to 0.50%

Carbon (C) enhances the hardenability and increases the strength of thesteel material. C also promotes spheroidization of carbides duringtempering in the production process, and increases the SSC resistance ofthe steel material. If the carbides are dispersed, the strength of thesteel material increases further. These effects will not be obtained ifthe C content is too low. On the other hand, if the C content is toohigh, the toughness of the steel material will decrease and quenchcracking is liable to occur. Therefore, the C content is within therange of 0.20 to 0.50%. A preferable lower limit of the C content is0.23%, and more preferably is 0.25%. A preferable upper limit of the Ccontent is 0.45%, and more preferably is 0.40.

Si: 0.05 to 0.50%

Silicon (Si) deoxidizes the steel. If the Si content is too low, thiseffect is not obtained. On the other hand, if the Si content is toohigh, the SSC resistance of the steel material decreases. Therefore, theSi content is within the range of 0.05 to 0.50%. A preferable lowerlimit of the Si content is 0.15%, and more preferably is 0.20%. Apreferable upper limit of the Si content is 0.45%, and more preferablyis 0.40%.

Mn: 0.05 to 1.00%

Manganese (Mn) deoxidizes the steel material. Mn also enhances thehardenability of the steel material. If the Mn content is too low, theseeffects are not obtained. On the other hand, if the Mn content is toohigh, Mn segregates at grain boundaries together with impurities such asP and S. In such a case, the SSC resistance of the steel material willdecrease. Therefore, the Mn content is within a range of 0.05 to 1.00%4.A preferable lower limit of the Mn content is 0.25%, and more preferablyis 0.30%. A preferable upper limit of the Mn content is 0.90%, and morepreferably is 0.80%.

P: 0.030% or less

Phosphorous (P) is an impurity. In other words, the P content is morethan 0%. P segregates at the grain boundaries and decreases the SSCresistance of the steel material. Therefore, the P content is 0.030% orless. A preferable upper limit of the P content is 0.020%, and morepreferably is 0.015%. Preferably, the P content is as low as possible.However, if the P content is excessively reduced, the production costincreases significantly. Therefore, when taking industrial productioninto consideration, a preferable lower limit of the P content is0.0001%, more preferably is 0.0002%, and further preferably is 0.003%.

S: 0.0100% or less

Sulfur (S) is an impurity. In other words, the S content is more than0%. S segregates at the grain boundaries and decreases the SSCresistance of the steel material. Therefore, the S content is 0.0100% orless. A preferable upper limit of the S content is 0.0050%, and morepreferably is 0.0030%. Preferably, the S content is as low as possible.However, if the S content is excessively reduced, the production costincreases significantly. Therefore, when taking industrial productioninto consideration, a preferable lower limit of the S content is0.0001%, more preferably is 0.0002%, and further preferably is 0.0003%.

Al: 0.005 to 0.100%

Aluminum (Al) deoxidizes the steel material. If the Al content is toolow, this effect is not obtained and the SSC resistance of the steelmaterial decreases. On the other hand, if the Al content is too high,coarse oxide-based inclusions are formed and the SSC resistance of thesteel material decreases. Therefore, the Al content is within a range of0.005 to 0.100%. A preferable lower limit of the Al content is 0.015%,and more preferably is 0.020%. A preferable upper limit of the Alcontent is 0.080%, and more preferably is 0.060%. In the presentdescription, the “Al” content means “acid-soluble Al”, that is, thecontent of “sol. Al”.

Cr: 0.10 to 1.50%

Chromium (Cr) enhances the hardenability of the steel material. Cr alsoincreases temper softening resistance of the steel material and enableshigh-temperature tempering. As a result, the SSC resistance of the steelmaterial increases. If the Cr content is too low, these effects are notobtained. On the other hand, if the Cr content is too high, thetoughness and SSC resistance of the steel material decreases. Therefore,the Cr content is within a range of 0.10 to 1.50%. A preferable lowerlimit of the Cr content is 0.25%, and more preferably is 0.30%. Apreferable upper limit of the Cr content is 1.30%.

Mo: 0.25 to 1.50%

Molybdenum (Mo) enhances the hardenability of the steel material. Moalso forms fine carbides and increases the temper softening resistanceof the steel material. As a result, Mo increases the SSC resistance ofthe steel material by high temperature tempering. If the Mo content istoo low, these effects are not obtained. On the other hand, if the Mocontent is too high, the aforementioned effects are saturated.Therefore, the Mo content is within a range of 0.25 to 1.50%. Apreferable lower limit of the Mo content is 0.50%, and more preferablyis 0.65%. A preferable upper limit of the Mo content is 1.30%, and morepreferably is 1.25%.

Ti: 0.002 to 0.050%

Titanium (Ti) forms nitrides, and refines crystal grains by the pinningeffect. As a result, the strength of the steel material increases. Ifthe Ti content is too low, this effect is not obtained. On the otherhand, if the Ti content is too high, Ti nitrides coarsen and the SSCresistance of the steel material decreases. Therefore, the Ti content iswithin a range of 0.002 to 0.050%. A preferable lower limit of the Ticontent is 0.003%, and more preferably is 0.005%. A preferable upperlimit of the Ti content is 0.030%, and more preferably is 0.020%.

N: 0.0100% or less

Nitrogen (N) is unavoidably contained. In other words, the N content ismore than 0%. N combines with Ti to form fine nitrides, and refinescrystal grains of the steel material by the pinning effect. However, ifthe N content is too high, N will form coarse nitrides and the SSCresistance of the steel material will decrease. Therefore, the N contentis 0.0100% or less. A preferable upper limit of the N content is0.0050%, and more preferably is 0.0040%. To obtain the above effect moreeffectively, a preferable lower limit of the N content is 0.0005%, morepreferably is 0.0010%, and further preferably is 0.0020%.

O: 0.0100% or less

Oxygen (O) is an impurity. In other words, the O content is more than0%. O forms coarse oxides and reduces the corrosion resistance of thesteel material. Therefore, the O content is 0.0100% or less. Apreferable upper limit of the O content is 0.0030%, and more preferablyis 0.0020%. Preferably, the O content is as low as possible. However, ifthe O content is excessively reduced, the production cost increasessignificantly. Therefore, when taking industrial production intoconsideration, a preferable lower limit of the O content is 0.0001%,more preferably is 0.0002%, and further preferably is 0.0003%.

The balance of the chemical composition of the steel material accordingto the present embodiment is Fe and impurities. Here, the term“impurities” refers to elements which, during industrial production ofthe steel material, are mixed in from ore or scrap that is used as a rawmaterial of the steel material, or from the production environment orthe like, and which are allowed within a range that does not adverselyaffect the steel material according to the present embodiment.

[Regarding Optional Elements]

The chemical composition of the steel material described above mayfurther contain B in lieu of a part of Fe.

B: 0 to 0.0050%

Boron (B) is an optional element, and need not be contained. In otherwords, the B content may be 0%. If contained B dissolves in the steel,enhances the hardenability of the steel material and increases the steelmaterial strength. If even a small amount of B is contained, this effectis obtained to a certain extent. However, if the B content is too high,coarse nitrides form and the SSC resistance of the steel materialdecreases. Therefore, the B content is within a range of 0 to 0.0050%. Apreferable lower limit of the B content is more than 0%, more preferablyis 0.0001%, further preferably is 0.0003%, and further preferably is0.0007%. In a case where it is intended to obtain a yield strength of758 MPa or more, it is preferable that the steel material contains B inan amount of 0.0001% or more. When B is contained in an amount of0.0001% or more, the yield strength of the steel material is stably made758 MPa or more. Therefore, in a case where the yield strength is withina range of 758 to less than 862 MPa, a preferable lower limit of the Bcontent is 0.0001%, more preferably is 0.0003%, and further preferablyis 0.0007%. A preferable upper limit of the B content is 0.0030%, andmore preferably is 0.0025%.

The chemical composition of the steel material described above mayfurther contain one or more types of element selected from the groupconsisting of V and Nb in lieu of a part of Fe. Each of these elementsis an optional element, and increases the SSC resistance of the steelmaterial.

V: 0 to 0.30%

Vanadium (V) is an optional element, and need not be contained. In otherwords, the V content may be 0%. If contained, V combines with C or N toform carbides, nitrides or carbo-nitrides and the like (hereinafter,referred to as “carbo-nitrides and the like”). These carbo-nitrides andthe like refine the substructure of the steel material by the pinningeffect, and improve the SSC resistance of the steel. V also forms finecarbides during tempering. The fine carbides increase the tempersoftening resistance of the steel material, and increase the strength ofthe steel material. In addition, because V also forms spherical MC-typecarbides, V suppresses the formation of acicular M₂C-type carbides andthereby increases the SSC resistance of the steel material. If even asmall amount of V is contained, these effects are obtained to a certainextent. However, if the V content is too high, the toughness of thesteel material decreases. Therefore, the V content is within the rangeof 0 to 0.30%. A preferable lower limit of the V content is more than0%, more preferably is 0.01%, and further preferably is 0.02%. Apreferable upper limit of the V content is 0.20%, more preferably is0.15%, and further preferably is 0.12%.

Nb: 0 to 0.100%

Niobium (Nb) is an optional element, and need not be contained. In otherwords, the Nb content may be 0%. If contained, Nb forms carbo-nitridesand the like. These carbo-nitrides and the like refine the substructureof the steel material by the pinning effect, and increase the SSCresistance of the steel material. In addition, because Nb also formsspherical MC-type carbides, Nb suppresses the formation of acicularM₂C-type carbides and thereby increases the SSC resistance of the steelmaterial. If even a small amount of Nb is contained, these effects areobtained to a certain extent. However, if the Nb content is too high,carbo-nitrides and the like are excessively formed and the SSCresistance of the steel material decreases. Therefore, the Nb content iswithin the range of 0 to 0.100%. A preferable lower limit of the Nbcontent is more than 0%, more preferably is 0.002%, further preferablyis 0.003%, and further preferably is 0.007%. In a case where it isintended to obtain a yield strength of 758 MPa or more, it is preferablethat the steel material contains Nb in an amount of 0.002% or more. WhenNb is contained in an amount of 0.002% or more, the yield strength ofthe steel material is stably made 758 MPa or more. Therefore, in a casewhere the yield strength is within a range of 758 to less than 862 MPa,a preferable lower limit of the Nb content is 0.002%, more preferably is0.003%, and further preferably is 0.007%. A preferable upper limit ofthe Nb content is less than 0.050%, more preferably is 0.035%, andfurther preferably is 0.030%.

The chemical composition of the steel material described above mayfurther contain one or more types of element selected from the groupconsisting of Ca. Mg, Zr and rare earth metal (REM) in lieu of a part ofFe. Each of these elements is an optional element, and increases the SSCresistance of the steel material.

Ca: 0 to 0.0100%

Calcium (Ca) is an optional element, and need not be contained. In otherwords, the Ca content may be 0%. If contained, Ca renders S in the steelmaterial harmless by forming sulfides, and thereby increases the SSCresistance of the steel material. If even a small amount of Ca iscontained, this effect is obtained to a certain extent. However, if theCa content is too high, oxides in the steel material coarsen and the SSCresistance of the steel material decreases. Therefore, the Ca content iswithin the range of 0 to 0.0100%. A preferable lower limit of the Cacontent is more than 0%, more preferably is 0.0001%, further preferablyis 0.0003%, and further preferably is 0.0006%, and even furtherpreferably is 0.0010%. A preferable upper limit of the Ca content is0.0025%, and more preferably is 0.0020%.

Mg: 0 to 0.0100%

Magnesium (Mg) is an optional element, and need not be contained. Inother words, the Mg content may be 0%. If contained, Mg renders S in thesteel material harmless by forming sulfides, and thereby increases theSSC resistance of the steel material. If even a small amount of Mg iscontained, this effect is obtained to a certain extent. However, if theMg content is too high, oxides in the steel material coarsen anddecrease the SSC resistance of the steel material. Therefore, the Mgcontent is within the range of 0 to 0.0100%. A preferable lower limit ofthe Mg content is more than 0%, more preferably is 0.0001%, furtherpreferably is 0.0003%, and further preferably is 0.0006%, and evenfurther preferably is 0.0010%. A preferable upper limit of the Mgcontent is 0.0025%, and more preferably is 0.0020′.

Zr 0 to 0.0100%

Zirconium (Zr) is an optional element, and need not be contained. Inother words, the Zr content may be 0%. If contained, Zr renders S in thesteel material harmless by forming sulfides, and thereby increases theSSC resistance of the steel material. If even a small amount of Zr iscontained, this effect is obtained to a certain extent. However, if theZr content is too high, oxides in the steel material coarsen, anddecreases the SSC resistance of the steel material. Therefore, the Zrcontent is within the range of 0 to 0.0100%. A preferable lower limit ofthe Zr content is more than 0%, more preferably is 0.0001%, furtherpreferably is 0.0003%, and further preferably is 0.0006%, and evenfurther preferably is 0.0010%. A preferable upper limit of the Zrcontent is 0.0025%, and more preferably is 0.0020%.

Rare earth metal (REM): 0 to 0.0100%

Rare earth metal (REM) is an optional element, and need not becontained. In other words, the REM content may be 0%. If contained, REMrenders S in the steel material harmless by forming sulfides, andthereby increases the SSC resistance of the steel material. REM alsocombines with P in the steel material and suppresses segregation of P atthe crystal grain boundaries. Therefore, a decrease in the SSCresistance of the steel material that is attributable to segregation ofP is suppressed. If even a small amount of REM is contained, theseeffects are obtained to a certain extent. However, if the REM content istoo high, oxides coarsen and the SSC resistance of the steel materialdecrease. Therefore, the REM content is within the range of 0 to0.0100%. A preferable lower limit of the REM content is more than 0%,more preferably is 0.0001%, further preferably is 0.0003%, and furtherpreferably is 0.0006%. A preferable upper limit of the REM content is0.0025%, and more preferably is 0.0020%.

Note that, in the present description the term “REM” refers to one ormore types of element selected from a group consisting of scandium (Sc)which is the element with atomic number 21, yttrium (Y) which is theelement with atomic number 39, and the elements from lanthanum (La) withatomic number 57 to lutetium (Lu) with atomic number 71 that arelanthanoids. Further, in the present description the term “REM content”refers to the total content of these elements.

In a case where two or more types of element selected from theaforementioned group containing Ca, Mg, Zr and REM are contained incombination, the total of the contents of these elements is preferably0.0100% or less, and more preferably is 0.0050% or less.

The chemical composition of the steel material described above mayfurther contain one or more types of element selected from the groupconsisting of Co and W in lieu of a part of Fe. Each of these elementsis an optional element that forms a protective corrosion coating in thesour environment and suppresses hydrogen penetration to the steelmaterial. By this means, each of these elements increases the SSCresistance of the steel material.

Co: 0 to 0.50%

Cobalt (Co) is an optional element, and need not be contained. In otherwords, the Co content may be 0%. If contained, Co forms a protectivecorrosion coating in the sour environment and suppresses hydrogenpenetration to the steel material. By this means, Co increases the SSCresistance of the steel material. If even a small amount of Co iscontained, this effect is obtained to a certain extent. However, if theCo content is too high, the hardenability of the steel material willdecrease, and the steel material strength will decrease. Therefore, theCo content is within the range of 0 to 0.50%. A preferable lower limitof the Co content is more than 0%, more preferably is 0.02%, and furtherpreferably is 0.05%. A preferable upper limit of the Co content is0.45%, and more preferably is 0.40%.

W: 0 to 0.50%

Tungsten (W) is an optional element, and need not be contained. In otherwords, the W content may be 0%. If contained, W forms a protectivecorrosion coating in the sour environment and suppresses hydrogenpenetration to the steel material. By this means, W increases the SSCresistance of the steel material. If even a small amount of W iscontained, this effect is obtained to a certain extent. However, if theW content is too high, coarse carbides form in the steel material andthe SSC resistance of the steel material decreases. Therefore, the Wcontent is within the range of 0 to 0.50%. A preferable lower limit ofthe W content is more than 0%, more preferably is 0.02%, and furtherpreferably is 0.05%. A preferable upper limit of the W content is 0.45%,and more preferably is 0.40%.

The chemical composition of the steel material described above mayfurther contain one or more types of element selected from the groupconsisting of Ni and Cu in lieu of a part of Fe. Each of these elementsis an optional element, and increases the hardenability of the steelmaterial.

Ni: 0 to 0.50%

Nickel (Ni) is an optional element, and need not be contained. In otherwords, the Ni content may be 0%. If contained, Ni enhances thehardenability of the steel material and increases the steel materialstrength. If even a small amount of Ni is contained, this effect isobtained to a certain extent. However, if the Ni content is too high,the Ni will promote local corrosion, and the SSC resistance of the steelmaterial will decrease. Therefore, the Ni content is within the range of0 to 0.50%. A preferable lower limit of the Ni content is more than 0%,and more preferably is 0.02%. A preferable upper limit of the Ni contentis 0.35%, and more preferably is 0.25%.

Cu: 0 to 0.50%

Copper (Cu) is an optional element, and need not be contained. In otherwords, the Cu content may be 0%. If contained, Cu enhances thehardenability of the steel material and increases the steel materialstrength. If even a small amount of Cu is contained, this effect isobtained to a certain extent. However, if the Cu content is too high,the hardenability of the steel material will be too high, and the SSCresistance of the steel material will decrease. Therefore, the Cucontent is within the range of 0 to 0.50%. A preferable lower limit ofthe Cu content is more than 0%, and more preferably is 0.02%. Apreferable upper limit of the Cu content is 0.35%, and more preferablyis 0.25%.

[Amount of Dissolved C]

The steel material according to the present embodiment contains anamount of dissolved C which is within the range of 0.010 to 0.050 mass%. If the amount of dissolved C is less than 0.010 mass %, theimmobilization of dislocations in the steel material will beinsufficient and excellent SSC resistance will not be obtained. On theother hand, if the amount of dissolved C is more than 0.050 mass %,conversely, the SSC resistance of the steel material will decrease.Therefore, the amount of dissolved Cis within the range of 0.010 to0.050 mass %. A preferable lower limit of the amount of dissolved C is0.020 mass % and more preferably is 0.030 mass %.

An amount of dissolved C within the aforementioned range is obtained by,for example, controlling the holding time in the tempering process andcontrolling the cooling rate in the tempering process. The reason is asdescribed hereunder.

The amount of dissolved C is highest immediately after quenching.Immediately after quenching, C is dissolved except for a small amountthereof that precipitated as carbides during quenching. In the temperingprocess thereafter, some of the C precipitates as carbides as a resultof being held for tempering. As a result, the amount of dissolved Cdecreases toward the thermal equilibrium concentration with respect tothe tempering temperature. If the holding time for tempering is tooshort, this effect will not be obtained and the amount of dissolved Cwill be too high. On the other hand, if the holding time for temperingis too long, the amount of dissolved C will approach the aforementionedthermal equilibrium concentration, and will hardly change. Therefore, inthe present embodiment, the holding time during tempering is within therange of 10 to 180 minutes.

If the cooling rate during cooling after tempering in the temperingprocess is slow, the dissolved C will reprecipitate while thetemperature is decreasing. In the conventional methods for producingsteel material, because cooling after tempering has been performed byallowing the steel material to cool, the cooling rate has been slow.Consequently, the amount of dissolved C has been almost 0 masse.Therefore, in the present embodiment, the cooling rate after temperingis raised, and a dissolved C amount within the range of 0.010 to 0.050mass % is obtained.

The cooling method is, for example, a method that performs forcedcooling of the steel material continuously from the temperingtemperature to thereby continuously decrease the surface temperature ofthe steel material. Examples of this kind of continuous coolingtreatment include a method that cools the steel material by immersion ina water bath, and a method that cools the steel material in anaccelerated manner by shower water cooling, mist cooling or forced aircooling.

The cooling rate after tempering is measured at a region that is mostslowly cooled within a cross-section of the steel material that istempered (for example, in the case of forcedly cooling both surfaces,the cooling rate is measured at the center portion of the steel materialthickness). Specifically, in a case where the steel material is a steelplate, the cooling rate after tempering can be determined based on atemperature measured by a sheath-type thermocouple that is inserted intothe center portion of the thickness of the steel plate. In a case wherethe steel material is a steel pipe, the cooling rate after tempering canbe determined based on a temperature measured by a sheath-typethermocouple that is inserted into the center portion of the wallthickness of the steel pipe. Further, in a case of forcedly cooling onlya surface on one side of the steel material, the cooling rate aftertempering can be determined based on the surface temperature on thenon-forcedly cooled side of the steel material that is measured by meansof a non-contact type infrared thermometer.

The temperature region from 600° C. to 200° C. is a temperature regionin which diffusion of C is comparatively fast. On the other hand, if thecooling rate after tempering is too fast, very little of the C that haddissolved after being held during tempering precipitates. Consequently,in some cases the amount of dissolved C is excessive. Therefore, in thepresent embodiment, the average cooling rate in the temperature regionfrom 600° C. to 200° C. is made 5 to 100° C./sec.

According to this method, in the steel material according to the presentembodiment, the amount of dissolved C can be made to fall within therange of 0.010 to 0.050 mass %. However, the amount of dissolved C inthe steel material may be adjusted to within the range of 0.010 to 0.050mass % by another method.

[Method for Calculating Amount of Dissolved C]

The term “amount of dissolved C” means the difference between the amountof C (mass % o) in carbides in the steel material and the C content ofthe chemical composition of the steel material. The amount of C incarbides in the steel material is determined by Formula (1) to Formula(5) using an Fe concentration <Fe>, a Cr concentration <Cr>a, an Mnconcentration <Mn>a, an Mo concentration <Mo>a, a V concentration <V>aand an Nb concentration <Nb>a in carbides (cementite and MC-typecarbides) obtained as residue when extraction residue analysis isperformed on the steel material, and an Fe concentration <Fe>b, a Crconcentration <Cr>b, an Mn concentration <Mn>b and an Mo concentration<Mo>b in cementite obtained by performing point analysis by EDS withrespect to cementite identified by performing TEM observation of areplica film obtained by an extraction replica method.<Mo>c=(<Fe>a+<Cr>a+<Mn>a)×<Mo>b/(<Fe>b+<Cr>b+<Mn>b)  (1)<Mo>d=<Mo>a−<Mo>c  (2)<C>a=(<Fe>a/55.85+<Cr>a/52+<Mn>a/53.94+<Mo>c/95.9)/3×12  (3)<C>b=(<V>a/50.94+<Mo>d/95.9+<Nb>a/92.9)×12  (4)(amount of dissolved C)=<C>−(<C>a+<C>b)  (5)

Note that, in the present description, the term “cementite” meanscarbides containing an Fe content of 50 mass % or more. Hereunder, themethod for calculating the amount of dissolved C is described in detail.

[Determination of C Content of Steel Material]

An analysis sample having the shape of a machined chip is taken from thesteel material. In a case where the steel material is a steel plate, ananalysis sample is taken from a center portion of the thickness. In acase where the steel material is a steel pipe, an analysis sample istaken from a center portion of the wall thickness. The C content (mass%) is analyzed by an oxygen-stream combustion-infrared absorptionmethod. The resulting value was taken to be the C content (<C>) of thesteel material.

[Calculation of C Amount that Precipitates as Carbides (Precipitated CAmount)]

The precipitated C amount is calculated by the following procedures 1 to4. Specifically, in procedure 1 an extraction residue analysis isperformed. In procedure 2, an extraction replica method using a TEM, andan element concentration analysis (hereunder, referred to as “EDSanalysis”) of elements in cementite is performed by EDS. In procedure 3,the Mo content is adjusted. In procedure 4, the precipitated C amount iscalculated.

[Procedure 1. Determination of Residual Amounts of Fe. Cr, Mn. Mo, V andNb by Extraction Residue Analysis]

In procedure 1, carbides in the steel material are captured as residue,and the contents of Fe, Cr, Mn, Mo, V and Nb in the residue aredetermined. Here, the term “carbides” is a generic term for cementite(M₃C-type carbides) and MC-type carbides. The specific procedure is asfollows. A cylindrical test specimen having a diameter of 6 mm and alength of 50 mm is extracted from the steel material. In a case wherethe steel material is a steel plate, the cylindrical test specimen isextracted from a center portion of the thickness in a manner so that thecenter of the thickness becomes the center of the cross-section. In acase where the steel material is a steel pipe, the cylindrical testspecimen is extracted from a center portion of the wall thickness of thesteel pipe in a manner so that the center of the wall thickness becomesthe center of the cross-section. The surface of the cylindrical testspecimen is polished to remove about 50 μm by preliminaryelectropolishing to obtain a newly formed surface. The electropholishedtest specimen is subjected to electrolysis in an electrolyte solution(10% acetylacetone+1% tetra-ammonium+methanol). The electrolyte solutionafter electrolysis is passed through a 0.2-μm filter to capture residue.The obtained residue is subjected to acid decomposition, and theconcentrations of Fe, Cr, Mn, Mo, V and Nb are determined in units ofmass percent by ICP (inductively coupled plasma) optical emissionspectrometry. The concentrations are defined as <Fe>a, <Cr>a, <Mn>a,<Mo>a, <V>a and <Nb>a, respectively.

[Procedure 2. Determination of Content of Fe, Cr, Mn and Mo in Cementiteby Extraction Replica Method and EDS]

In procedure 2, the content of each of Fe, Cr, Mn and Mo in cementite isdetermined. The specific procedure is as follows. A micro test specimenis cut out from the steel material. In a case where the steel materialis a steel plate, the micro test specimen is cut out from a centerportion of the thickness. In a case where the steel material is a steelpipe, the micro test specimen is cut out from a center portion of thewall thickness of the steel pipe. The surface of the micro test specimenis finished by mirror polishing. The test specimen is immersed for 10minutes in a 3% nital etching reagent to etch the surface. The corrodedsurface is covered with a carbon deposited film. The test specimen whosesurface is covered with the deposited film is immersed in a 5% nitaletching reagent, and held therein for 20 minutes to cause the depositedfilm to peel off. The deposited film that peeled off is cleaned withethanol, and thereafter is scooped up with a sheet mesh and dried. Thedeposited film (replica film) is observed using a TEM, and pointanalysis by EDS is performed with respect to 20 particles of cementite.The concentration of each of Fe, Cr, Mn and Mo is determined in units ofmass percent when taking the total of the alloying elements excludingcarbon in the cementite as 100%. The concentrations are determined for20 particles of cementite, and the arithmetic average values for therespective elements are defined as <Fe>b, <Cr>b, <Mn>b and <Mo>b.

[Procedure 3. Adjustment of Mo Amount]

Next, the Mo concentration in the carbides is determined. In this case,Fe, Cr, Mn and Mo concentrate in cementite. On the other hand, V, Nb andMo concentrate in MC-type carbides. In other words, Mo is caused toconcentrate in both cementite and MC-type carbides by tempering.Therefore, the Mo amount is calculated separately for cementite and forMC-type carbides. Note that, in some cases a part of V also concentratesin cementite. However, the amount of V that concentrates in cementite isnegligibly small in comparison to the amount of V that concentrates inMC-type carbides. Therefore, when determining the amount of dissolved C,V is regarded as concentrating only in MC-type carbides.

Specifically, the amount of Mo precipitating as cementite (<Mo>c) iscalculated by Formula (1).<Mo>c=(<Fe>a+<Cr>a+<Mn>a)×<Mo>b/(<Fe>b+<Cr>b+<Mn>b)  (1)

On the other hand, the amount of Mo precipitating as MC-type carbides(<Mo>d) is calculated in units of mass percent by Formula (2).<Mo>d=<Mo>a−<Mo>c  (2)

[Procedure 4. Calculation of Precipitated C Amount]

The precipitated C amount is calculated as the total of the C amountprecipitating as cementite (<C>a) and the C amount precipitating asMC-type carbides (<C>b). <C>a and <C>b are calculated in units of masspercent by Formula (3) and Formula (4), respectively. Note that, Formula(3) is a formula that is derived from the fact that the structure ofcementite is a M₃C type structure (M include Fe. Cr, Mn and Mo).<C>a=(<Fe>a/55.85+<Cr>a/52+<Mn>a/53.94+<Mo>c/95.9)/3×12  (3)<C>b=(<V>a/50.94+<Mo>d/95.9+<Nb>a/92.9)×12  (4)

Thus, the precipitated C amount is <C>a+<C>b.

[Calculation of Amount of Dissolved C]

The amount of dissolved C (hereunder, also referred to as “<C>c”) iscalculated in units of mass percent by Formula (5) as a differencebetween the C content (<C>) and the precipitated C amount of the steelmaterial.<C>c=<C>−(<C>a+<C>b)  (5)

[Microstructure]

The microstructure of the steel material according to the presentembodiment is principally composed of tempered martensite and temperedbainite. More specifically, the volume ratio of tempered martensiteand/or tempered bainite in the microstructure is 90% or more. In otherwords, the total of the volume ratios of tempered martensite andtempered bainite in the microstructure is 90% or more. The balance ofthe microstructure is, for example, ferrite or perlite. If themicrostructure of the steel material containing the aforementionedchemical composition contains tempered martensite and tempered bainitein an amount equivalent to a total volume ratio of 90% or more, theyield strength will be within the range of 655 to less than 862 MPa (95ksi grade or 110 ksi grade), and the yield ratio will be 85% or more.

In the present embodiment, if the yield strength is within the range of655 to less than 862 MPa (95 ksi grade or 110 ksi grade) and the yieldratio is 85% or more, it is assumed that the volume ratios of temperedmartensite and tempered bainite in the microstructure is 90% or more.Preferably, the microstructure is composed of only tempered martensiteand/or tempered bainite. In other words, the volume ratios of temperedmartensite and tempered bainite in the microstructure may be 100%.

Note that, the following method can be adopted in the case ofdetermining the volume ratios of tempered martensite and temperedbainite by observation. In a case where the steel material is a steelplate, a test specimen having an observation surface with dimensions of10 mm in the rolling direction and 10 mm in the thickness direction iscut out from a center portion of the thickness. In a case where thesteel material is a steel pipe, a test specimen having an observationsurface with dimensions of 10 mm in the pipe axis direction and 8 mm inthe wall thickness direction is cut out from a center portion of thewall thickness.

After polishing the observation surface of the test specimen to obtain amirror surface, the test specimen is immersed for about 10 seconds in anital etching reagent, to reveal the microstructure by etching. Theetched observation surface is observed by means of a secondary electronimage obtained using a scanning electron microscope (SEM), andobservation is performed for 10 visual fields. The area of each visualfield is 400 μm² (magnification of ×5000).

In each visual field, tempered martensite and tempered bainite areidentified based on the contrast. The total of the area fractions oftempered martensite and tempered bainite that are identified isdetermined. In the present embodiment, the arithmetic average value ofthe totals of the area fractions of tempered martensite and temperedbainite determined in all visual fields is taken as the volume ratio oftempered martensite and tempered bainite.

[Shape of Steel Material]

The shape of the steel material according to the present embodiment isnot particularly limited. The steel material is, for example, a steelpipe or a steel plate. In a case where the steel material is an oil-wellsteel pipe, preferably the steel material is a seamless steel pipe. In acase where the steel material is an oil-well steel pipe, the wallthickness is not particularly limited. A preferable wall thickness is 9to 60 mm. The steel material according to the present embodiment is, inparticular, suitable for use as a heavy-wall oil-well steel pipe. Morespecifically, even if the steel material according to the presentembodiment is an oil-well steel pipe having a thick wall of 15 mm ormore or, furthermore, 20 mm or more, the steel material exhibitsexcellent strength and SSC resistance.

[Yield Strength and Yield Ratio of Steel Material]

The yield strength of the steel material according to the presentembodiment is within a range of 655 to less than 862 MPa (95 ksi gradeor 110 ksi grade), and the yield ratio of the steel material is 85% ormore. In short, the steel material according to the present embodimenthas a yield strength of 95 ksi grade or 110 ksi grade, and a yield ratioof 85% or more.

The yield strength of the steel material according to the presentembodiment is defined in accordance with ASTM E8 (2013). Specifically,the yield strength of the steel material according to the presentembodiment is defined for each range of yield strength. Morespecifically, in a case where a yield strength of the steel materialaccording to the present embodiment is within a range of 655 to lessthan 758 MPa (95 ksi grade), the term “yield strength” means the stresswhen elongation of 0.5% is obtained in a tensile test (0.5% yieldstress). In a case where a yield strength of the steel materialaccording to the present embodiment is within a range of 758 to lessthan 862 MPa (110 ksi grade), the term “yield strength” means the stresswhen elongation of 0.7% is obtained in a tensile test (0.7% yieldstress).

Even though the steel material according to the present embodiment has ayield strength within the range of 655 to less than 862 MPa (95 ksigrade or 110 ksi grade), the steel material also has excellent SSCresistance by satisfying the conditions regarding the chemicalcomposition, amount of dissolved C and microstructure, which aredescribed above.

The yield strength of the steel material according to the presentembodiment can be determined by the following method. A tensile test isperformed in accordance with ASTM E8 (2013). A round bar test specimenis taken from the steel material according to the present embodiment. Ina case where the steel material is a steel plate, the round bar testspecimen is taken from the center portion of the thickness. In a casewhere the steel material is a steel pipe, the round bar test specimen istaken from the center portion of the wall thickness. Regarding the sizeof the round bar test specimen, for example, the round bar test specimenhas a parallel portion diameter of 6.35 mm and a parallel portion lengthof 35.6 mm. Note that the axial direction of the round bar test specimenis parallel to the rolling direction of the steel material. A tensiletest is performed in the atmosphere at normal temperature (25° C.) usingthe round bar test specimen.

In a case where the stress at the time of 0.5% elongation (0.5% yieldstress) obtained in the tensile test is within a range of 655 to lessthan 758 MPa (95 ksi grade), 0.5% yield stress is defined as the yieldstrength. In a case where the stress at the time of 0.7% elongation(0.7% yield stress) obtained in the tensile test is within a range of758 to less than 862 MPa (110 ksi grade) 0.7% yield stress is defined asthe yield strength. Further, the largest stress during uniformelongation is defined as the tensile strength (MPa). The yield ratio(YR)(%) can be obtained by a ratio of the yield strength (YS) to thetensile strength (TS)(YR=YS/TS).

The steel material according to the present embodiment contains theaforementioned chemical composition, contains an amount of dissolved Cwithin the range of 0.010 to 0.050 mass %, has a yield strength within arange of 655 to less than 862 MPa, and has a yield ratio of 85% or more.In this respect, when steel materials contain the chemical compositionaccording to the present embodiment and the same microstructure(phases), the dislocation density is considered to be the dominantfactor that determines the yield strength.

Specifically, the dislocation density of a steel material that containsthe aforementioned chemical composition, contains an amount of dissolvedC within the range of 0.010 to 0.050 mass %, has a yield strength withina range of 655 to less than 758 MPa (95 ksi grade), and has a yieldratio of 85% or more is within the range of 1.0×10¹⁴ to less than4.4×10¹⁴ (m⁻²). Furthermore, the dislocation density of a steel materialthat contains the aforementioned chemical composition, contains anamount of dissolved C within the range of 0.010 to 0.050 mass %, has ayield strength within a range of 758 to less than 862 MPa (110 ksigrade), and has a yield ratio of 85% or more is within the range of4.4×10¹⁴ to less than 6.5×10¹⁴ (m⁻²).

On the other hand, the dislocation density of a steel material thatcontains the aforementioned chemical composition, contains an amount ofdissolved C within the range of 0.010 to 0.050 mass %, has a yieldstrength within a range of 862 to less than 965 MPa (125 ksi grade), andhas a yield ratio of 85% or more is within the range of 6.5×10¹⁴ to lessthan 9.2×10¹⁴ (m⁻²). The steel material according to the presentembodiment contains the aforementioned chemical composition, theaforementioned amount of dissolved C, the aforementioned yield strengthand the aforementioned yield ratio. Consequently, the dislocationdensity of the steel material according to the present embodimentdiffers from the dislocation density of a steel material that containsthe aforementioned chemical composition, the aforementioned amount ofdissolved C and the aforementioned yield ratio, but has a differentyield strength.

The dislocation density of the steel material according to the presentembodiment can be determined by the following method. A test specimenfor use for dislocation density measurement is taken from the steelmaterial according to the present embodiment. In a case where the steelmaterial is a steel plate, the test specimen is taken from a centerportion of the thickness. In a case where the steel material is a steelpipe, the test specimen is taken from a center portion of the wallthickness. The size of the test specimen is, for example, 20 mm width×20mm length×2 mm thickness. The thickness direction of the test specimenis the thickness direction of the steel material (plate thicknessdirection or wall thickness direction). In this case, the observationsurface of the test specimen is a surface having a size of 20 mm inwidth×20 mm in length.

The observation surface of the test specimen is mirror-polished, andfurthermore electropolishing is performed using a 10 vol % perchloricacid (acetic acid solvent) solution to remove strain in the outer layer.The observation surface after the treatment is subjected to X-raydiffraction (XRD) to determine the half-value width ΔK of the peaks ofthe (110), (211) and (220) planes of the body-centered cubic structure(iron).

In the XRD, measurement of the half-value width ΔK is performed byemploying CoKα lines as the X-ray source, 30 kV as the tube voltage, and100 mA as the tube current. In addition, LaB₆ (lanthanum hexaboride)powder is used in order to measure a half-value width originating fromthe X-ray diffractometer.

The non-uniform strain E of the test specimen is determined based on thehalf-value width ΔK determined by the aforementioned method and theWilliamson-Hall equation (Formula (6)).ΔK×cos θ/λ=0.9/D+2ε×sin θ/λ  (6)

In Formula (6), 0 represents the diffraction angle, λ represents thewavelength of the X-ray, and D represents the crystallite diameter.

In addition, the dislocation density p (m⁻²) can be determined using theobtained non-uniform strain s and Formula (7).ρ=14.4×ε² /b ²  (7)

In Formula (7), b represents the Burgers vector (b=0.248 (nm)) of thebody-centered cubic structure (iron).

[SSC Resistance of Steel Material]

As described above, there is a possibility that dislocations willocclude hydrogen. That is, it has been considered that the higher theyield strength of the steel material is, the less the SSC resistance ofthe steel material that is obtained. Therefore, according to presentembodiment, excellent SSC resistance is defined for each yield strength.Specifically, excellent SSC resistance is defined as follows.

[SSC Resistance when Yield Strength is 95 Ksi Grade]

In a case where the yield strength of the steel material is of 95 ksigrade, the SSC resistance of the steel material can be evaluated by aDCB test performed in accordance with “Method D” described in NACETM0177-2005, and a low temperature SSC resistance test performed inaccordance with “Method A” described in NACE TM0177-2005.

The DCB test is performed in accordance with “Method D” described inNACE TM0177-2005. Specifically, a mixed aqueous solution containing 5.0mass % of sodium chloride and 0.5 mass % of acetic acid (NACE solutionA) is employed as the test solution. A DCB test specimen illustrated inFIG. 2A is taken from the steel material according to the presentembodiment. In a case where the steel material is a steel plate, thetest specimen is taken from a center portion of the thickness. In a casewhere the steel material is a steel pipe, the test specimen is takenfrom a center portion of the wall thickness. The longitudinal directionof the DCB test specimen is parallel with the rolling direction of thesteel material. A wedge illustrated in FIG. 2B is also taken from thesteel material according to the present embodiment. A thickness t of thewedge is made 3.12 (mm).

Referring to FIG. 2A, the aforementioned wedge is driven in between thearms of the DCB test specimen. The DCB test specimen into which thewedge was driven is then enclosed inside a test vessel. Thereafter, theaforementioned test solution is poured into the test vessel so as toleave a vapor phase portion, and is adopted as the test bath. After thetest bath is degassed, 1 atm H₂S gas is blown into the test vessel tomake the test bath a corrosive environment. The inside of the testvessel is held at a temperature of 24° C. for two weeks (336 hours)while stirring the test bath. After being held for two weeks, the DCBtest specimen is taken out from the test vessel.

A pin is inserted into a hole formed in the tip of the arms of each DCBtest specimen that is taken out and a notch portion is opened with atensile testing machine, and a wedge releasing stress P is measured. Inaddition, the notch in the DCB test specimen is released in liquidnitrogen, and a crack propagation length “a” with respect to crackpropagation that occurred during immersion is measured. The crackpropagation length “a” is measured visually using vernier calipers. Afracture toughness value K_(ISSC) (MPa√m) is determined using Formula(8) based on the obtained wedge releasing stress P and the crackpropagation length “a”.

$\begin{matrix}{K_{ISSC} = \frac{{{Pa}\left( {{2\sqrt{3}} + 2.38^{\frac{h}{a}}} \right)}\left( \frac{B}{Bn} \right)^{\frac{1}{\sqrt{3}}}}{{Bh}^{\frac{3}{2}}}} & (8)\end{matrix}$

In Formula (8), h represents the height (mm) of each arm of the DCB testspecimen, B represents the thickness (mm) of the DCB test specimen, andBn represents the web thickness (mm) of the DCB test specimen. These aredefined in “Method D” of NACE TM0177-2005.

The low temperature SSC resistance test is performed in accordance with“Method A” described in NACE TM177-2005. Round bar test specimens aretaken from the steel material according to the present embodiment. In acase where the steel material is a steel plate, the round bar testspecimens are taken from a center portion of the thickness. In a casewhere the steel material is a steel pipe, the round bar test specimensare taken from a center portion of the wall thickness. The size of theround bar test specimen is, for example, 6.35 mm in diameter, with aparallel portion length of 25.4 mm. The axial direction of the round bartest specimen is parallel to the rolling direction of the steelmaterial.

A mixed aqueous solution containing 5.0 mass % of sodium chloride and0.5 mass % of acetic acid (NACE solution A) is employed as the testsolution. A stress equivalent to 95% of the actual yield stress isapplied to the round bar test specimen. The test solution at 4° C. ispoured into a test vessel so that the round bar test specimen to whichthe stress has been applied is immersed therein, and this is adopted asa test bath. After degassing the test bath, 1 atm H₂S gas is blown intothe test bath and is caused to saturate in the test bath. The test bathinto which 1 atm H₂S gas was blown is held for 720 hours (30 days) at 4°C.

In the steel material according to the present embodiment, in a casewhere the yield strength of the steel material is of 95 ksi grade, thefracture toughness value K_(ISSC) of determined under the foregoing DCBtest is 42.0 MPa√m or more, and cracking is not confined after 720 hours(30 days) elapses at the condition under the foregoing low temperatureSSC resistance test.

[SSC Resistance when Yield Strength is 110 Ksi Grade]

In a case where the yield strength of the steel material is of 110 ksigrade, the SSC resistance of the steel material can be evaluated by aDCB test performed in accordance with “Method D” described in NACETM0177-2005. The DCB test is performed in a similar manner to theaforementioned DCB test that is performed when the yield strength is of95 ksi grade except that the thickness t of the wedge is made 2.92 (mm).

In the steel material according to the present embodiment, when theyield strength of the steel material is of 110 ksi grade, the fracturetoughness value K_(ISSC) of determined under the foregoing DCB test is27.5 MPa√m or more.

[Production Method]

A method for producing the steel material according to the presentembodiment will now be described. The production method describedhereunder is a method for producing a seamless steel pipe as one exampleof the steel material according to the present embodiment. The methodfor producing a seamless steel pipe includes a process of preparing ahollow shell (preparation process), and a process of subjecting thehollow shell to quenching and tempering to obtain a seamless steel pipe(quenching process and tempering process). Note that, a method forproducing the steel material according to the present embodiment is notlimited to the production method described hereunder. Each of theseprocesses is described in detail hereunder.

[Preparation Process]

In the preparation process, an intermediate steel material containingthe aforementioned chemical composition is prepared. The method is notparticularly limited as long as the intermediate steel material containsthe aforementioned chemical composition. As used here, the term“intermediate steel material” refers to a plate-shaped steel material ina case where the end product is a steel plate, and refers to a hollowshell in a case where the end product is a steel pipe.

The preparation process may include a process in which a startingmaterial is prepared (starting material preparation process), and aprocess in which the starting material is subjected to hot working toproduce an intermediate steel material (hot working process). Hereunder,a case in which the preparation process includes the starting materialpreparation process and the hot working process is described in detail.

[Starting Material Preparation Process]

In the starting material preparation process, a starting material isproduced using molten steel containing the aforementioned chemicalcomposition. The method for producing the starting material is notparticularly limited, and a well-known method can be used. Specifically,a cast piece (a slab, bloom or billet) is produced by a continuouscasting process using the molten steel. An ingot may also be produced byan ingot-making process using the molten steel. As necessary, the slab,bloom or ingot may be subjected to blooming to produce a billet. Thestarting material (a slab, bloom or billet) is produced by the abovedescribed process.

[Hot Working Process]

In the hot working process, the starting material that was prepared issubjected to hot working to produce an intermediate steel material. In acase where the steel material is a seamless steel pipe, the intermediatesteel material corresponds to a hollow shell. First, the billet isheated in a heating furnace. Although the heating temperature is notparticularly limited, for example, the heating temperature is within arange of 1100 to 1300° C. The billet that is extracted from the heatingfurnace is subjected to hot working to produce a hollow shell (seamlesssteel pipe). The method of performing the hot working is notparticularly limited, and a well-known method can be used.

For example, the Mannesmann process may be performed as the hot workingto produce the hollow shell. In this case, a round billet ispiercing-rolled using a piercing machine. When performingpiercing-rolling, although the piercing ratio is not particularlylimited, the piercing ratio is, for example, within a range of 1.0 to4.0. The round billet that underwent piercing-rolling is furtherhot-rolled to form a hollow shell using a mandrel mill, a reducer, asizing mill or the like. The cumulative reduction of area in the hotworking process is, for example, 20 to 70%.

A hollow shell may also be produced from the billet by another hotworking method. For example, in the case of a heavy-wall steel materialof a short length such as a coupling, a hollow shell may be produced byforging such as Ehrhardt process. A hollow shell is produced by theabove process. Although not particularly limited, the wall thickness ofthe hollow shell is, for example, 9 to 60 mm.

The hollow shell produced by hot working may be air-cooled (as-rolled).The hollow shell produced by hot working may be subjected to directquenching after hot working without being cooled to normal temperature,or may be subjected to quenching after undergoing supplementary heating(reheating) after hot working.

In a case of performing direct quenching or quenching aftersupplementary heating, it is preferable to stop the cooling midwaythrough the quenching process and conduct slow cooling. In this case,quenching cracking can be suppressed. In a case where direct quenchingis performed after hot working, or quenching is performed aftersupplementary heating after hot working, a stress relief treatment (SRtreatment) may be performed at a time that is after quenching and beforethe heat treatment of the next process. In this case, residual stress ofthe hollow shell can be eliminated.

As described above, an intermediate steel material is prepared in thepreparation process. The intermediate steel material may be produced bythe aforementioned preferable process, or may be an intermediate steelmaterial that was produced by a third party, or an intermediate steelmaterial that was produced in another factory other than the factory inwhich a quenching process and a tempering process that are describedlater are performed, or at a different works. The quenching process isdescribed in detail hereunder.

[Quenching Process]

In the quenching process, the intermediate steel material (hollow shell)that was prepared is subjected to quenching. In the present description,the term “quenching” means rapidly cooling the intermediate steelmaterial that is at a temperature not less than the A₃ point. Apreferable quenching temperature is 850 to 1000° C. In a case wheredirect quenching is performed after hot working, the quenchingtemperature corresponds to the surface temperature of the intermediatesteel material that is measured by a thermometer placed on the exit sideof the apparatus that performs the final hot working. Further, in a casewhere quenching is performed after supplementary heating is performedafter hot working, the quenching temperature corresponds to thetemperature of the furnace that performs the supplementary heating.

If the quenching temperature is too high, in some cases prior-y grainsbecome coarse and the SSC resistance of the steel material decreases.Therefore, a quenching temperature in the range of 850 to 1000° C. ispreferable.

The quenching method, for example, continuously cools the intermediatesteel material (hollow shell) from the quenching starting temperature,and continuously decreases the surface temperature of the hollow shell.The method of performing the continuous cooling treatment is notparticularly limited, and a well-known method can be used. The method ofperforming the continuous cooling treatment is, for example, a methodthat cools the hollow shell by immersing the hollow shell in a waterbath, or a method that cools the hollow shell in an accelerated mannerby shower water cooling or mist cooling.

If the cooling rate during quenching is too slow, the microstructuredoes not become one that is principally composed of martensite andbainite, and the mechanical property defined in the present embodiment(that is, the yield strength of 95 ksi grade or 110 ksi grade, and theyield ratio of 85% or more) is not obtained.

Therefore, as described above, in the method for producing the steelmaterial according to the present embodiment, the intermediate steelmaterial is rapidly cooled during quenching. Specifically, in thequenching process, the average cooling rate when the surface temperatureof the intermediate steel material (hollow shell) is within the range of800 to 500° C. during quenching is defined as a cooling rate duringquenching CR₈₀₀₋₅₀₀. More specifically, the cooling rate duringquenching CR₈₀₀₋₅₀₀ is determined based on a temperature that ismeasured at a region that is most slowly cooled within a cross-sectionof the intermediate steel material that is being quenched (for example,in the case of forcedly cooling both surfaces, the cooling rate ismeasured at the center portion of the thickness of the intermediatesteel material).

A preferable of the cooling rate during quenching CR₈₀₀₋₅₀₀ is 300°C./min or higher. A more preferable lower limit of the cooling rateduring quenching CR₈₀₀₋₅₀₀ is 450° C./min, and further preferably is600° C./min. Although an upper limit of the cooling rate duringquenching CR₈₀₀₋₅₀₀ is not particularly defined, for example, the upperlimit is 60000° C./min.

Preferably, quenching is performed after performing heating of thehollow shell in the austenite zone a plurality of times. In this case,the SSC resistance of the steel material increases because austenitegrains are refined prior to quenching. Heating in the austenite zone maybe repeated a plurality of times by performing quenching a plurality oftimes, or heating in the austenite zone may be repeated a plurality oftimes by performing normalizing and quenching. Quenching and temperingdescribed below may be performed in combination a plurality of times.Therefore, both quenching and tempering may be performed a plurality oftimes. In this case, the SSC resistance of the steel material furtherincreases. Hereunder, the tempering process will be described in detail.

[Tempering Process]

In the tempering process, tempering is performed after performing theaforementioned quenching. In the present description, the term“tempering” means reheating the intermediate steel material afterquenching to a temperature that is not more than the Ai point andholding the intermediate steel material at that temperature. Thetempering temperature is appropriately adjusted in accordance with thechemical composition of the steel material and the yield strength, whichis to be obtained. Here, the tempering temperature corresponds to thetemperature of the furnace when the intermediate steel material afterquenching is heated and held at the relevant temperature.

That is, in the tempering process according to the present embodiment,with respect to the intermediate steel material (hollow shell)containing the aforementioned chemical composition, the temperingtemperature is adjusted so as to adjust the yield strength of the steelmaterial to within a range of 655 to less than 862 MPa (95 ksi grade or110 ksi grade). Hereunder, in a case where it is intended to obtain ayield strength of 95 ksi grade and 110 ksi grade, the temperingtemperature is described in detail.

[Tempering Temperature when Yield Strength is 95 Ksi Grade]

When it is intended to obtain a yield strength of 95 ksi grade (655 toless than 758 MPa), a preferable tempering temperature is in a rangefrom 650 to 740° C. If the tempering temperature is too high, in somecases the dislocation density is reduced too much and a yield strengthof 95 ksi grade cannot be obtained. In contrast, if the temperingtemperature is too low, in some cases the dislocation density cannot beadequately reduced. In such cases, the yield strength of the steelmaterial becomes too high and/or the SSC resistance of the steelmaterial decreases.

Accordingly, in a case where it is intended to obtain a yield strengthof 95 ksi grade, it is preferable to set the tempering temperaturewithin the range of 650 to 740° C. When it is intended to obtain a yieldstrength of 95 ksi grade, a more preferable lower limit of the temperingtemperature is 670° C., and further preferably is 680° C. When it isintended to obtain a yield strength of 95 ksi grade, a more preferableupper limit of the tempering temperature is 730° C. and furtherpreferably is 720° C.

[Tempering Temperature when Yield Strength is 110 Ksi Grade]

When it is intended to obtain a yield strength of 110 ksi grade (758 toless than 862 MPa), a preferable tempering temperature is in a rangefrom 650 to 720° C. If the tempering temperature is too high, in somecases the dislocation density is reduced too much and a yield strengthof 110 ksi grade cannot be obtained. In contrast, if the temperingtemperature is too low, in some cases the dislocation density cannot beadequately reduced. In such cases, the yield strength of the steelmaterial becomes too high and/or the SSC resistance of the steelmaterial decreases.

Accordingly, in a case where it is intended to obtain a yield strengthof 110 ksi grade, it is preferable to set the tempering temperaturewithin the range of 650 to 720° C. When it is intended to obtain a yieldstrength of 110 ksi grade, a more preferable lower limit of thetempering temperature is 660° C., and further preferably is 670° C. Whenit is intended to obtain a yield strength of 110 ksi grade, a morepreferable upper limit of the tempering temperature is 715° C. andfurther preferably is 710° C.

As described above, in the tempering process according to the presentembodiment, the tempering temperature is appropriately controlled inaccordance with the yield strength which it is intended to obtain (95ksi grade and 110 ksi grade). A person skilled in the art will besufficiently capable of making the yield strength of the steel materialcontaining the aforementioned chemical composition fall within theintended range by appropriately adjusting the undermentioned holdingtime at the aforementioned tempering temperature.

In the tempering process according to the present embodiment, apreferable holding time (tempering time) is within the range of 10 to180 minutes. Here, the tempering time (holding time) means the period oftime from the loading of the intermediate steel material into the heattreatment furnace till the extracting.

If the tempering time is too short, the amount of dissolved C becomesexcessive because precipitation of carbides does not proceed. Even ifthe tempering time is overlong, there will be almost no change in theamount of dissolved C. Therefore, in order to control the amount ofdissolved C to be within an appropriate range, the preferable temperingtime is within a range of 10 to 180 minutes.

A more preferable lower limit of the tempering time is 15 minutes. Amore preferable upper limit of the tempering time is 120 minutes, andfurther preferably is 90 minutes. Note that, in a case where the steelmaterial is a steel pipe, in comparison to other shapes, temperaturevariations with respect to the steel pipe are liable to occur duringholding for tempering. Therefore, in a case where the steel material isa steel pipe, the tempering time is preferably set within a range of 15to 180 minutes.

[Regarding Rapid Cooling after Tempering]

Conventionally, cooling after tempering has not been controlled.However, the temperature region from 600° C. to 200° C. is a temperatureregion in which diffusion of C is comparatively fast. Therefore, if thecooling rate of the steel material after tempering (that is, after beingheld for the aforementioned holding time at the aforementioned temperingtemperature) is slow, almost all of the C that had dissolved willreprecipitate while the temperature is decreasing. In other words, theamount of dissolved C will be approximately 0 mass %. Therefore, in thepresent embodiment, the intermediate steel material (hollow shell) aftertempering is rapidly cooled.

Specifically, in the tempering process, the average cooling rate whenthe temperature of the intermediate steel material (hollow shell) iswithin the range of 600 to 200° C. after tempering is defined as acooling rate after tempering CR₆₀₀₋₂₀₀ In the method for producing thesteel material according to the present embodiment, the cooling rateafter tempering CR₆₀₀₋₂₀₀ is preferably 5° C./sec or higher. On theother hand, if the cooling rate after tempering is too fast, in somecases very little of the C that had dissolved will precipitate, and theamount of dissolved C will be excessive. In such a case, the SSCresistance of the steel material decreases.

Therefore, in the present description, the preferable cooling rate aftertempering CR₆₀₀₋₂₀₀ is within the range of 5 to 100° C./sec. A morepreferable lower limit of the cooling rate after tempering CR₆₀₀₋₂₀₀ is10° C./sec, and further preferably is 15° C./sec. A more preferableupper limit of the cooling rate after tempering CR₆₀₀₋₂₀₀ is 50° C./sec.and further preferably is 40° C./sec. Note that, in the case when thetempering is performed a plurality of times, it may be controlled thecooling after the final tempering. That is, the cooling after thetempering except for the final tempering may be performed as same asconventional manner.

A method for cooling so that the cooling rate after tempering CR₆₀₀₋₂₀₀is within the range of 5 to 100° C./sec is not particularly limited, anda well-known method can be used. The cooling method, for example, is amethod that performs forced cooling of a hollow shell continuously fromthe tempering temperature to thereby continuously decrease the surfacetemperature of the hollow shell. Examples of this kind of continuouscooling treatment include a method that cools the hollow shell byimmersion in a water bath, and a method that cools the hollow shell inan accelerated manner by shower water cooling, mist cooling or forcedair cooling. Note that, the cooling rate after tempering CR₆₀₀₋₂₀₀ ismeasured at a region that is most slowly cooled within a cross-sectionof the intermediate steel material that is tempered (for example, in thecase of forcedly cooling both surfaces, the cooling rate is measured atthe center portion of the thickness of the intermediate steel material).

The steel material according to the present embodiment can be producedby the production method that is described above. A method for producinga seamless steel pipe has been described as one example of theaforementioned production method. However, the steel material accordingto the present embodiment may be a steel plate or another shape. Anexample of a method for producing a steel plate or a steel material ofanother shape also includes, for example, a preparation process, aquenching process and a tempering process, similarly to the productionmethod described above. However, the aforementioned production method isone example, and the steel material according to the present embodimentmay be produced by another production method.

Hereunder, the present invention is described more specifically by wayof examples.

Example 1

In Example 1, the SSC resistance of a steel material having a yieldstrength of 95 ksi grade (655 to less than 758 MPa) was investigated.Specifically, molten steels having the chemical compositions shown inTable 1 were produced.

TABLE 1 Test Chemical Composition (Unit is mass %; balance is Fe andimpurities) Number C Si Mn P S Al Cr Mo Ti N O B 1-A 0.24 0.29 0.820.006 0.0030 0.026 1.00 0.25 0.008 0.0039 0.0017 — 1-B 0.26 0.23 0.420.008 0.0030 0.037 1.02 0.45 0.026 0.0033 0.0013 0.0011 1-C 0.27 0.300.45 0.005 0.0020 0.037 0.50 1.20 0.007 0.0032 0.0014 0.0011 1-D 0.240.28 0.26 0.001 0.0024 0.030 0.87 1.25 0.002 0.0040 0.0015 — 1-E 0.280.24 0.60 0.003 0.0027 0.042 0.90 0.84 0.008 0.0045 0.0013 — 1-F 0.270.33 0.17 0.013 0.0021 0.020 0.97 0.80 0.023 0.0035 0.0014 0.0012 1-G0.24 0.33 0.67 0.015 0.0029 0.030 1.03 0.72 0.005 0.0015 0.0012 — 1-H0.28 0.26 0.15 0.012 0.0019 0.040 0.96 0.96 0.011 0.0032 0.0031 — 1-I0.28 0.30 0.52 0.002 0.0028 0.030 0.85 0.66 0.021 0 0015 0.0012 — 1-J0.27 0.34 0.77 0.003 0.0023 0.040 1.04 0.83 0.006 0.0025 0.0012 — 1-K0.24 0.34 0.63 0.014 0.0025 0.030 0.99 1.21 0.004 0.0025 0.0013 — 1-L0.28 0.34 0.26 0.015 0.0019 0.030 1.03 0.84 0.017 0.0038 0.0014 — 1-M0.26 0.22 0.44 0.012 0.0004 0.036 1.05 0.47 0.027 0.0041 0.0013 0.00121-N 0.27 0.30 0.42 0.007 0.0011 0.040 1.04 0.71 0.006 0.0019 0.00100.0013 1-O 0.27 0.29 0.46 0.001 0.0015 0.040 0.97 1.15 0.002 0.00250.0012 — 1-P 0.28 0.23 0.56 0.002 0.0012 0.030 0.98 1.20 0.006 0.00250.0015 — 1-Q 0.27 0.25 0.94 0.011 0.0022 0.026 0.52 0.20 0.006 0.00320.0014 — Test Chemical Composition (Unit is mass %; balance is Fe andimpurities) Number V Nb Ca Mg Zr REM Co W Cu Ni 1-A — — — — — — — — — —1-B 0.10 0.027 0.0013 — — — — — — — 1-C 0.10 0.030 0.0014 — — — — — — —1-D 0.10 0.027 — — — — — — — — 1-E — 0.031 — — — — — — — — 1-F — 0.027 —— — — — — — — 1-G — 0.025 0.0012 — — — — — — — 1-H — 0.025 — 0.0013 — —— — — — 1-I — 0.027 — — 0.0010 — — — — — 1-J — 0.027 — — — 0.0012 — — —— 1-K 0.09 0.027 — — — — 0.50 — — — 1-L — 0.027 — — — — — 0.50 — — 1-M —0.027 — 0.0030 — — — — 0.02 — 1-N 0.10 0.030 — — — 0.0012 0.10 — — 0.031-O 0.09 0.025 — — — — — — 0.03 — 1-P 0.09 0.025 — — — — — — — 0.03 1-Q— 0.027 0.0013 — — — — — — —

A billet having an outer diameter of 310 mm was produced using theaforementioned molten steel. The produced billet was heated to 1250° C.,and then the billet was hot rolled to produce a seamless steel pipehaving an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.A sample material having a thickness of 13.84 mm in a plate shape wastaken from the produced seamless steel pipe such that the samplematerial has a size enough for taking out specimens for use inevaluation tests, which will be described later.

The obtained the sample material of each test number was subjected toquenching and tempering. Specifically, the quenching and tempering wereperformed once on the sample materials of test numbers excluding theTest Numbers 1-16 and 1-25. On the sample materials of the Test Numbers1-16 and 1-25, the quenching and tempering were repeated twice.

In more specific terms, the sample materials of test numbers excludingthe Test Numbers 1-16 and 1-25 were held at a quenching temperature of920° C. for 30 minutes. After being held, the sample materials of testnumbers excluding the Test Numbers 1-16 and 1-25 were immersed in awater bath for water cooling. At this time, the cooling rate duringquenching (CR₈₀₀₋₅₀₀) was 300° C./min. The quenching temperature was setto the temperature of the furnace that performed the heating ofquenching. The cooling rate during quenching (CR₈₀₀₋₅₀₀) was determinedfrom the temperature that was measured by a type K thermocouple of asheath type being inserted into a center portion of the sample materialin advance.

After quenching, the sample materials of test numbers excluding the TestNumbers 16 and 25 were subjected to tempering. For the tempering, atempering temperature was adjusted such that the yield strength of 95ksi grade (655 to less than 758 MPa) was achieved. Specifically, in thetempering performed on the sample materials of test numbers excludingthe Test Numbers 1-16 and 1-25, the tempering temperature (° C.) and thetempering time (min) were as shown in Table 2.

After performing the heat treatment at the respective temperingtemperatures, the sample materials of test numbers excluding the TestNumbers 1-16 and 1-25 were cooled. For the cooling, the sample materialswere subjected to a controlled cooling by mist water cooling. Thetempering temperature was set to the temperature of the furnace thatperformed the tempering. The cooling rate after tempering (CR₆₀₀₋₂₀₀)was determined from the temperature that was measured by a type Kthermocouple of a sheath type being inserted into a center portion ofthe sample material in advance. In the tempering performed on the samplematerials of test numbers excluding the Test Numbers 1-16 and 1-25, thecooling rate after tempering (CR₆₀₀₋₂₀₀)(° C./sec) was as shown in Table2.

On the sample materials of the Test Numbers 1-16 and 1-25, the quenchingand tempering were repeated twice as described above. The samplematerials of the Test Numbers 1-16 and 1-25 were held at a quenchingtemperature of 920° C. for 10 minutes. The sample materials of the TestNumbers 1-16 and 1-25 after being held were immersed in a water bath forwater cooling. At this time, the cooling rate during first quenching(CR₈₀₀₋₅₀₀) was 300° C./min. The sample materials of the Test Numbers1-16 and 1-25 subjected to quenching were held at a temperingtemperature of 700° C. for 30 minutes of tempering time. The samplematerials of the Test Numbers 1-16 and 1-25 after being held wereallowed to cool to a normal temperature.

After performing the first quenching and tempering, the sample materialsof the Test Numbers 1-16 and 1-25 were subjected to the secondquenching. Specifically, the sample materials of the Test Numbers 1-16and 1-25 were held at a quenching temperature of 900° C. for 30 minutes.The sample materials of the Test Numbers 1-16 and 1-25 after being heldwere immersed in a water bath for water cooling. At this time, thecooling rate during second quenching (CR₈₀₀₋₅₀₀) was 300° C./min.

For the second tempering, as with the case of the sample materials oftest numbers excluding the Test Numbers 1-16 and 1-25, the temperingtemperature was adjusted such that the yield strength of 95 ksi grade(655 to less than 758 MPa) was achieved. Specifically, in the secondtempering performed on the sample materials of the Test Numbers 1-16 and1-25, the tempering temperature (° C.) and the tempering time (min) wereas shown in Table 2.

After being subjected to the tempering, the sample materials of the TestNumbers 1-16 and 1-25 were cooled. For the cooling, the sample materialswere subjected to a controlled cooling by mist water cooling. Thecooling rate after tempering (CR₆₀₀₋₂₀₀)(° C./sec) of the samplematerials of the Test Numbers 1-16 and 1-25 was as shown in Table 2. Forthe sample materials of the Test Numbers 1-16 and 1-25, the temperatureof the furnace that performed the heating of quenching was also used forthe quenching temperature. Similarly, the temperature of the furnacethat performed the tempering was also used for the temperingtemperature. Further, the cooling rates during first and secondquenching (CR₈₀₀₋₅₀₀) were determined from the temperature that wasmeasured by a type K thermocouple of a sheath type being inserted into acenter portion of the sample material in advance. Similarly, the coolingrates after first and second tempering (CR₆₀₀₋₂₀₀) were determined fromthe temperature that was measured by a type K thermocouple of a sheathtype being inserted into a center portion of the sample material inadvance.

TABLE 2 Low Dissolved temperature Tempering Tempering C Dislocation SSCK_(1SSC) (MPa√m) Test Temperature Time CR₆₀₀₋₂₀₀ YS TS YR Amount Densityresistant Average Number Steel (° C.) (min) (° C./sec) (MPa) (MPa) (%)(mass %) (×10¹⁴ × m⁻²) test 1 2 3 Value 1-1 1-A 660 60 19.0 747 858 870.026 4.1 E 48.1 47.3 44.1 46.5 1-2 1-A 680 60 19.2 685 809 85 0.029 3.8E 47.9 42.9 43.5 44.8 1-3 1-B 690 60 19.8 746 853 88 0.026 4.1 E 43.850.0 46.9 46.9 1-4 1-C 710 60 19.7 756 844 90 0.037 4.3 E 42.4 42.9 44.543.3 1-5 1-D 720 30 22.0 699 794 88 0.011 3.9 E 49.0 45.6 42.0 45.5 1-61-E 710 60 25.0 689 807 85 0.027 3.7 E 43.4 46.8 47.9 46.0 1-7 1-F 71060 18.4 695 800 87 0.033 3.8 E 48.6 47.2 48.1 48.0 1-8 1-G 710 60 19.6693 796 87 0.015 3.6 E 47.0 42.5 46.5 45.3 1-9 1-H 710 60 20.1 693 79387 0.034 3.5 E 46.7 49.5 46.5 47.6 1-10 1-I 710 60 21.5 683 793 86 0.0393.4 E 42.2 46.6 42.0 43.6 1-11 1-J 710 60 34.9 692 791 88 0.035 3.5 E43.6 47.7 47.6 46.3 1-12 1-K 720 30 19.2 697 808 86 0.015 3.6 E 42.345.7 44.1 44.0 1-13 1-L 700 60 32.4 688 803 86 0.044 3.3 E 48.7 45.343.2 45.7 1-14 1-M 700 30 20.1 692 788 88 0.026 3.6 E 51.9 48.9 45.448.7 1-15 1-M 710 30 19.8 665 763 87 0.018 3.1 E 53.5 48.7 50.9 51.01-16 1-N 730 30 20.1 659 712 93 0.039 2.7 E 49.6 48.6 52.0 50.0 1-17 1-O720 30 34.9 695 806 86 0.044 3.7 E 48.8 42.2 44.9 45.3 1-18 1-P 720 3036.4 681 790 86 0.029 3.5 E 50.0 43.0 50.0 47.7 1-19 1-A 670 60 0.05 729847 86 0.008 4.0 NA 32.3 35.1 37.8 35.1 1-20 1-A 680 60 0.05 680 802 850.002 3.4 NA 35.1 31.9 35.9 34.3 1-21 1-B 690 60 0.10 742 860 86 0.0084.3 NA 31.8 31.8 30.0 31.2 1-22 1-C 710 60 0.10 756 847 89 0.003 4.2 NA38.3 32.3 32.5 34.4 1-23 1-M 700 30 0.10 693 793 87 0.006 3.6 NA 38.339.5 40.1 39.3 1-24 1-M 710 30 0.40 667 760 88 0.008 3.2 NA 40.9 40.140.1 40.4 1-25 1-N 730 30 0.50 658 725 91 0.005 3.1 NA 37.6 39.7 39.739.0 1-26 1-A 680 60 110 687 812 85 0.054 3.5 NA 35.8 34.5 35.1 35.11-27 1-Q 650 60 18.4 742 887 84 0.012 4.5 NA 30.6 34.8 30.5 32.0

Evaluation Results

A tensile test, microstructure determination test, amount of dissolved Cmeasurement test, a dislocation density measurement test. DCB test andlow temperature SSC resistance test described hereunder were performedon the sample materials of each test number after the aforementionedtempering.

[Tensile Test]

A tensile test was performed in accordance with ASTM E8(2013). Round bartensile test specimens having a diameter of 6.35 mm and a parallelportion length of 35 mm were taken from the center portion of thethickness of the sample materials of each test number. The axialdirection of each of the round bar test specimens was parallel to therolling direction of the sample material (the axial direction of theseamless steel pipe). A tensile test was performed in the atmosphere atnormal temperature (25° C.) using each test number of round bar testspecimens, and the yield strength (MPa) and tensile strength (MPa) wereobtained. Note that, in the present example, the stress at the time of0.5% elongation (0.5% yield stress) obtained in the tensile test definedas the yield strength for each test number. Further, the largest stressduring uniform elongation was taken as the tensile strength. A ratio ofthe yield strength to the tensile strength (YS/TS) was adopted as theyield ratio (%). The determined yield strength (YS), tensile strength(TS) and yield ratio (YR) are shown in Table 2.

[Microstructure Determination Test]

With respect to the microstructures of the sample materials of each testnumber, when the yield strength was in a range of 655 to less than 758MPa (95 ksi grade) and the yield ratio was 85% or more, it wasdetermined that the volume ratios of tempered martensite and temperedbainite was 90% or more. In the microstructure of the sample materialsof test numbers excluding the Test Number 1-27, the volume ratios oftempered martensite and tempered bainite was 90% or more.

[Amount of Dissolved C Measurement Test]

With respect to the sample materials of each test number, the amount ofdissolved C (mass %) was measured and calculated by the measurementmethod described above. Note that, the TEM used was JEM-2010manufactured by JEOL Ltd., the acceleration voltage was set to 200 kV.For the EDS point analysis the irradiation current was 2.56 nA, andmeasurement was performed for 60 seconds at each point. The observationregions for the TEM observation were 8 μm×8 μm, and observation wasperformed with respect to an arbitrary 10 visual fields. The residualamounts of each element and the concentrations of each element incementite that were used to calculate the amount of dissolved C were aslisted in Table 3.

TABLE 3 Residual Amount Concentration in Cementite Dissolved C Test(mass %) (mass %) Amount Number Steel Fe Cr Mn Mo V Nb Fe Cr Mn Mo (mass%) 1-1 1-A 2.2 0.46 0.11 0.21 — — 83.1 8.4 3.1 5.4 0.026 1-2 1-A 2.20.48 0.10 0.19 — — 80.8 10.8 2.5 5.9 0.029 1-3 1-B 2.3 0.37 0.10 0.240.065 0.027 82.7 8.5 2.6 6.1 0.026 1-4 1-C 2.2 0.23 0.10 0.36 0.0650.023 81.5 10.4 2.5 5.7 0.037 1-5 1-D 2.1 0.47 0.11 0.21 0.071 0.02184.8 7.7 2.7 4.8 0.011 1-6 1-E 2.5 0.42 0.11 0.35 — 0.021 83.7 9.0 2.44.9 0.027 1-7 1-F 2.4 0.40 0.12 0.30 — 0.022 81.9 9.6 2.6 5.9 0.033 1-81-G 2.0 0.50 0.15 0.34 — 0.023 83.8 7.7 2.7 5.8 0.015 1-9 1-H 2.5 0:430.15 0.27 — 0.022 83.1 8.9 2.9 5.2 0.034 1-10 1-I 2.4 0.49 0.15 0.27 —0.022 84.1 7.3 2.5 6.1 0.039 1-11 1-J 2.2 0.48 0.10 0.33 — 0.023 84.58.9 2.2 4.5 0.035 1-12 1-K 2.0 0.43 0.12 0.27 0.071 0.025 82.5 8.9 2.66.0 0.015 1-13 1-L 2.3 0.52 0.11 0.25 — 0.022 85.0 8.4 2.3 4.3 0.0441-14 1-M 2.2 0.50 0.10 0.33 — 0.023 81.6 10.4 2.5 5.6 0.026 1-15 1-M 2.20.56 0.11 0.35 — 0.028 84.3 7.3 2.9 5.5 0.018 1-16 1-N 2.1 0.45 0.110.24 0.061 0.024 83.3 10.2 2.4 4.0 0.039 1-17 1-O 2.0 0.43 0.13 0.250.071 0.024 83.2 8.9 2.8 5.1 0.044 1-18 1-P 2.2 0.47 0.10 0.36 0.0710.024 82.3 9.5 2.2 6.0 0.029 1-19 1-A 2.2 0.54 0.11 0.28 — — 85.9 7.22.9 4.0 0.008 1-20 1-A 2.3 0.56 0.11 0.26 — — 85.8 7.2 2.9 4.1 0.0021-21 1-B 2.5 0.42 0.10 0.21 0.061 0.028 79.0 14.0 2.8 4.2 0.008 1-22 1-C2.6 0.23 0.11 0.39 0.063 0.024 83.2 9.9 2.5 4.4 0.003 1-23 1-M 2.2 0.600.12 0.39 — 0.023 85.6 7.4 3.0 4.1 0.006 1-24 1-M 2.2 0.53 0.11 0.43 —0.026 82.4 11.0 2.1 4.5 0.008 1-25 1-N 2.6 0.47 0.11 0.23 0.063 0.02483.7 9.5 2.3 4.5 0.005 1-26 1-A 2.0 0.46 0.09 0.12 — — 80.3 10.1 3.3 6.30.054 1-27 1-Q 2.8 0.44 0.11 0.15 — 0.021 86.8 8.5 2.6 2.0 0.012

[Dislocation Density Measurement Test]

Test specimens for use for dislocation density measurement by theaforementioned method were taken from the sample material of each testnumber. In addition, the dislocation density (m⁻²) was determined by theaforementioned method. The determined dislocation density (×10¹⁴×m²) isshown in Table 2.

[DCB Test]

With respect to the sample materials of each test number, a DCB test wasconducted in accordance with “Method D” of NACE TM0177-2005, and the SSCresistance was evaluated. Specifically, three of the DCB test specimenillustrated in FIG. 2A were taken from a center portion of the thicknessof the sample material of each test number. The DCB test specimens weretaken in a manner such that the longitudinal direction of each DCB testspecimen was parallel with the rolling direction of the sample material(the axial direction of the seamless steel pipe). A wedge illustrated inFIG. 2B was further taken from the sample material of each test number.A thickness t of the wedge was 3.12 mm. The aforementioned wedge wasdriven in between the arms of the DCB test specimen.

A mixed aqueous solution containing 5.0 mass % of sodium chloride and0.5 mass % of acetic acid (NACE solution A) was used as the testsolution. The test solution was poured into the test vessel enclosingthe DCB test specimen into which the wedge had been driven inside so asto leave a vapor phase portion, and was adopted as the test bath. Afterthe test bath was degassed, 1 atm H₂S gas was blown into the test vesselto make the test bath a corrosive environment. The inside of the testvessel was held at a temperature of 24° C. for two weeks (336 hours)while stirring the test bath. After being held for two weeks, the DCBtest specimen was taken out from the test vessel.

A pin was inserted into a hole formed in the tip of the arms of each DCBtest specimen that was taken out and a notch portion was opened with atensile testing machine, and a wedge releasing stress P was measured. Inaddition, the notch in the DCB test specimen being immersed in the testbath was released in liquid nitrogen, and a crack propagation length “a”with respect to crack propagation that occurred during immersion wasmeasured. The crack propagation length “a” could be measured visuallyusing vernier calipers. A fracture toughness value K_(ISSC) (MPa√m) wasdetermined using Formula (8) based on the obtained wedge releasingstress P and the crack propagation length “a”. The arithmetic average ofthe three fracture toughness value K_(ISSC) (MPa√m) was determined, anddefined as the fracture toughness value K_(ISSC) (MPa√m) of the samplematerial of the relevant test number.

$\begin{matrix}{K_{ISSC} = \frac{{{Pa}\left( {{2\sqrt{3}} + 2.38^{\frac{h}{a}}} \right)}\left( \frac{B}{Bn} \right)^{\frac{1}{\sqrt{3}}}}{{Bh}^{\frac{3}{2}}}} & (8)\end{matrix}$

In Formula (8), h (mm) represents the height of each arm of the DCB testspecimen, B (mm) represents the thickness of the DCB test specimen, andBn (mm) represents the web thickness of the DCB test specimen. These aredefined in “Method D” of NACE TM0177-2005.

For the sample material of each test number, the obtained fracturetoughness values K_(ISSC) are shown in Table 2. If the fracturetoughness value K_(ISSC) that was defined as described above was 42.0MPa√m or more, it was determined that the SSC resistance was good. Notethat, the clearance between the anus when the wedge is driven in priorto immersion in the test bath influences the Kiss value. Accordingly,actual measurement of the clearance between the arms was performed inadvance using a micrometer, and it was also confirmed that the clearancewas within the range in the API standards.

[Low Temperature SSC Resistance Test]

With respect to the sample materials of each test number, a lowtemperature SSC resistance test was conducted in accordance with “MethodA” of NACE TM0177-2005, and the SSC resistance was evaluated.Specifically, three of the round bar test specimens each of which was6.35 mm in the diameter of a parallel portion and 25.4 mm in the lengthof the parallel portion, were taken from a center portion of thethickness of the sample material of each test number. The axialdirection of the round bar test specimen was parallel with the rollingdirection of the sample material (the axial direction of the seamlesssteel pipe). Tensile stress was applied in the axial direction of theround bar test specimens of each test number. At this time, the appliedstress was adjusted so as to be 95% of the actual yield stress of eachsample material in accordance with “Method A” of NACE TM0177-2005.

A mixed aqueous solution containing 5.0 mass % of sodium chloride and0.5 mass % of acetic acid (NACE solution A) was used as the testsolution. The test solution at 4° C. was poured into three test vessels,and these were adopted as test baths. The three round bar test specimensto which the stress was applied were immersed individually in mutuallydifferent test vessels as the test baths. After each test bath wasdegassed, 1 atm H₂S gas was blown into the respective test baths andcaused to saturate. The test baths in which the 1 atm H₂S gas wassaturated were held at 4° C. for 720 hours.

After immersion for 720 hours, the round bar test specimens of each testnumber were observed to determine whether or not sulfide stress cracking(SSC) had occurred. Specifically, after immersion for 720 hours, theround bar test specimens were observed with the naked eye. Samplematerials for which cracking was not confirmed in all three of the testspecimens as the result of the observation were determined as being “E”(Excellent). On the other hand, sample materials for which cracking wasconfirmed in at least one test specimen were determined as being “NA”(Not Acceptable).

[Test Results]

The test results are shown in Table 2.

Referring to Table 1 and Table 2, the chemical compositions of thesample materials of Test Numbers 1-1 to 1-18 were appropriate, the yieldstrength was in the range of 655 to less than 758 MPa (95 ksi grade),and the yield ratio was 85% or more. In addition, the amount ofdissolved C was in the range of 0.010 to 0.050 mass %. As a result, theK_(ISSC) value was 42.0 MPa√m or more. Further, the cracking was notconfirmed in all three of the test specimens in the low temperature SSCresistance test. In other words, excellent SSC resistance was exhibited.Note that, the dislocation density of the sample materials of TestNumbers 1-1 to 1-18 were within the range of 1.0×10¹⁴ to less than4.4×10¹⁴ m⁻².

On the other hand, for the sample materials of Test Numbers 1-19 to1-25, the cooling rate after tempering (CR₆₀₀₋₂₀₀) was too slow.Consequently, the amount of dissolved C was less than 0.010 mass %. As aresult, the fracture toughness value K_(ISSC) was less than 42.0 MPa√mand the cracking was confirmed in the low temperature SSC resistancetest. In other words, excellent SSC resistance was not exhibited. Notethat, the dislocation density of the sample materials of Test Numbers1-19 to 1-25 were within the range of 10×10¹⁴ to less than 4.4×10¹⁴ m⁻².

For the sample material of Test Number 1-26, the cooling rate aftertempering (CR₆₀₀₋₂₀₀) was too fast. Consequently, the amount ofdissolved C was more than 0.050 mass %. As a result, the fracturetoughness value K_(ISSC) was less than 42.0 MPa√m and the cracking wasconfirmed in the low temperature SSC resistance test. In other words,excellent SSC resistance was not exhibited. Note that, the dislocationdensity of the sample material of Test Number 1-26 was within the rangeof 1.0×10¹⁴ to less than 4.4×10⁴ m⁻².

For the sample material of Test Number 1-27, the Mo content was too low.As a result, the yield ratio was less than 85%. As a result, thefracture toughness value K_(ISSC) was less than 42.0 MPa m and thecracking was confirmed in the low temperature SSC resistance test. Inother words, excellent SSC resistance was not exhibited. Note that, thedislocation density of the sample material of Test Number 1-26 was4.4×10¹⁴ m⁻² or more.

Example 2

In Example 2, the SSC resistance of a steel material having a yieldstrength of 110 ksi guide (758 to less than 862 MPa) was investigated.Specifically, molten steels having the chemical compositions shown inTable 4 were produced.

TABLE 4 Test Chemical Composition (Unit is mass %; balance is Fe andimpurities) Number C Si Mn P S Al Cr Mo Ti N O B 2-A 0.27 0.28 0.490.008 0.0019 0.027 1.04 0.71 0.020 0.0029 0.0010 0.0012 2-B 0.27 0.300.42 0.007 0.0011 0.035 1.02 0.70 0.006 0.0025 0.0010 0.0013 2-C 0.270.30 0.42 0.007 0.0011 0.040 0.55 1.25 0.006 0.0025 0.0010 0.0013 2-D0.27 0.30 0.42 0.007 0.0011 0.040 3.03 0.65 0.006 0.0020 0.0010 0.00132-E 0.27 0.29 0.41 0.007 0.0010 0.035 0.96 1.34 0.006 0.0025 0.00100.0012 2-F 0.28 0.29 0.45 0.007 0.0010 0.035 0.51 1.06 0.004 0.00250.0010 0.0012 2-G 0.28 0.27 0.47 0.007 0.0010 0.035 0.50 1.26 0.0040.0025 0.0010 0.0012 2-H 0.25 0.34 0.35 0.007 0.0010 0.035 0.77 0.890.004 0.0025 0.0010 0.0012 2-I 0.26 0.33 0.30 0.007 0.0010 0.035 1.130.79 0.004 0.0025 0.0010 0.0010 2-J 0.29 0.25 0.37 0.007 0.0010 0.0351.15 0.87 0.004 0.0025 0.0010 0.0010 2-K 0.27 0.32 0.36 0.007 0.00100.035 1.12 1.22 0.004 0.0025 0.0010 0.0010 2-L 0.27 0.31 0.43 0.0070.0010 0.035 0.55 1.22 0.004 0.0025 0.0010 0.0010 2-M 0.27 0.33 0.360.007 0.0010 0.035 0.62 1.23 0.004 0.0025 0.0010 0.0030 2-N 0.27 0.310.34 0.007 0.0010 0.035 0.48 0.68 0.004 0.0025 0.0010 0.0010 2-O 0.270.28 0.48 0.007 0.0010 0.035 0.75 0.20 0.004 0.0025 0.0010 0.0010 TestChemical Composition (Unit is mass %; balance is Fe and impurities)Number V Nb Ca Mg REM Zr Co W Cu Ni 2-A — 0.028 — — — — — — — — 2-B 0.100.030 — — — — — — — — 2-C 0.10 0.030 0.0012 — — — — — 0.02 — 2-D 0.100.030 — — — — 0.10 0.10 — — 2-E 0.10 0.025 — — — — — — — — 2-F 0.100.028 — — — — — — 0.03 — 2-G 0.10 0.028 — — — — — — — 0.03 2-H 0.100.028 0.0012 — — — — — — — 2-I 0.10 0.025 — 0.0015 — — — — — — 2-J 0.100.025 — — 0.0012 — — — — — 2-K 0.10 0.025 — — — 0.0013 — — — — 2-L 0.100.025 — — — — 0.10 — — — 2-M 0.30 0.025 — — — — — 0.10 — — 2-N 0.100.025 0.0012 — — — 0.10 — — — 2-O — 0.025 — — — — — — — —

A billet having an outer diameter of 310 mm was produced using theaforementioned molten steel. The produced billet was heated to 1250° C.,and then the billet was hot rolled to produce a seamless steel pipehaving an outer diameter of 244.48 mm and a wall thickness of 13.84 mm.A sample material having a thickness of 13.84 mm in a plate shape wastaken from the produced seamless steel pipe such that the samplematerial has a size enough for taking out specimens for use inevaluation tests, which will be described later.

The obtained the sample material of each test number was subjected toquenching and tempering. Specifically, the quenching and tempering wererepeated twice on the sample materials of each test number. In morespecific terms, the sample materials of each test number were held at aquenching temperature of 920° C. for 10 minutes. After being held, thesample materials of each test number were immersed in a water bath forwater cooling. At this time, the cooling rate during first quenching(CR₈₀₀₋₅₀₀) was 300° C./min. The quenching temperature was set to thetemperature of the furnace that performed the heating of quenching. Thecooling rate during quenching (CR₈₀₀₋₅₀₀) was determined from thetemperature that was measured by a type K thermocouple of a sheath typebeing inserted into a center portion of the sample material in advance.

After first quenching, the sample materials of each test number weresubjected to first tempering. For the first tempering, the samplematerials of each test number were held at a tempering temperature of700° C. for 30 minutes of tempering time, and were allowed to cool to anormal temperature. Here, the temperature of the furnace that performedthe tempering was also used for the tempering temperature.

After performing the first quenching and tempering, the sample materialsof each test number were subjected to the second quenching.Specifically, the sample materials of each test number were held at aquenching temperature of 900° C. for 30 minutes. The sample materials ofeach test number after being held were immersed in a water bath forwater cooling. At this time, the cooling rate during second quenching(CR₈₀₀₋₅₀₀) was 300° C./min.

After performing the second quenching the sample materials of each testnumber were subjected to the second tempering. As with the case of thesample materials of each test number, the tempering temperature wasadjusted such that the yield strength of 110 ksi grade (758 to less than862 MPa) was achieved. Specifically, in the second tempering performedon the sample materials of each test number, the tempering temperature(° C.) and the tempering time (min) were as shown in Table 5.

After being subjected to the tempering, the sample materials of eachtest number were cooled. For the cooling, the sample materials of eachtest number were subjected to a controlled cooling by mist watercooling. The cooling rate after tempering (CR₆₀₀₋₂₀₀)(° C./sec) of thesample materials of each test number was as shown in Table 5. Note that,the cooling rates after first and second tempering (CR₆₀₀₋₂₀₀) weredetermined from the temperature that was measured by a type Kthermocouple of a sheath type being inserted into a center portion ofthe sample material in advance.

TABLE 5 Dissolved Tempering Tempering C Dislocation K_(1SSC) (MPa√m)Test Temperature Time CR₆₀₀₋₂₀₀ YS TS YR Amount Density Average NumberSteel (° C.) (min) (° C./sec) (MPa) (MPa) (%) (mass %) (×10¹⁴ × m⁻²) 1 23 Value 2-1 2-A 670 30 19.3 817 897 91 0.024 6.0 32.3 29.7 29.5 30.5 2-22-A 675 30 19.7 816 899 91 0.027 5.9 29.0 29.7 29.5 29.4 2-3 2-A 680 3020.0 798 883 90 0.037 5.0 34.1 32.2 32.2 32.8 2-4 2-B 700 30 20.5 800876 91 0.037 5.2 34.1 36.8 39.7 36.9 2-5 2-B 710 30 19.4 779 855 910.029 4.6 34.3 35.1 35.2 34.9 2-6 2-C 700 30 21.0 807 876 92 0.032 5.234.4 34.6 37.4 35.5 2-7 2-C 710 30 22.5 772 855 90 0.047 4.5 37.4 39.636.3 37.8 2-8 2-D 700 30 21.4 807 876 92 0.047 5.4 34.5 35.1 36.8 35.52-9 2-D 710 30 19.3 786 855 92 0.050 5.1 39.7 38.9 34.7 37.8 2-10 2-E710 30 32.8 774 835 93 0.022 4.8 34.8 35.6 35.3 35.2 2-11 2-F 710 3043.9 773 850 91 0.031 4.6 35.5 35.5 36.3 35.8 2-12 2-G 710 30 36.6 780846 92 0.033 4.8 35.1 36.5 34.7 35.4 2-13 2-H 710 30 38.6 772 855 900.012 4.6 343 34.4 36.1 34.9 2-14 2-I 710 30 39.6 780 856 91 0.025 4.735.7 34.5 35.2 35.1 2-15 2-J 710 30 35.2 762 843 90 0.048 4.5 35.2 36.036.0 35.7 2-16 2-K 710 30 38.4 775 850 91 0.016 4.7 34.2 35.8 36.7 35.62-17 2-L 710 30 44.4 762 826 92 0.017 4.6 35.2 34.9 36.9 35.7 2-18 2-M710 30 43.5 760 841 90 0.016 4.4 35.4 35.3 34.1 34.9 2-19 2-N 710 3039.6 774 842 92 0.028 4.8 35.6 34.1 35.9 35.2 2-20 2-A 670 30 0.4 813899 90 0.005 5.6 23.2 26.6 29.1 26.3 2-21 2-A 675 30 0.4 813 898 910.004 5.3 23.1 29.2 23.2 25.2 2-22 2-A 680 30 0.4 800 889 90 0.003 5.023.3 27.9 29.1 26.8 2-23 2-B 700 30 0.5 820 896 92 0.006 6.3 24.7 26.428.0 26.4 2-24 2-B 710 30 0.3 786 862 91 0.004 4.7 24.7 29.1 28.0 27.32-25 2-C 700 30 0.4 814 903 90 0.005 5.8 24.6 27.8 27.8 26.7 2-26 2-C710 30 0.4 779 862 90 0.004 4.7 24.7 29.2 25.6 26.5 2-27 2-D 700 30 0.3814 889 92 0.005 5.6 24.7 28.0 26.3 26.3 2-28 2-D 710 30 0.4 793 862 920.005 5.3 26.6 26.5 26.4 26.5 2-29 2-A 675 30 115.0 803 890 90 0.065 5.123.2 23.1 23.5 23.3 2-30 2-O 675 30 31.7 760 844 90 0.036 4.6 26.5 26.426.3 26.4

Evaluation Results

A tensile test, microstructure determination test, amount of dissolved Cmeasurement test, a dislocation density measurement test and DCB testdescribed hereunder were performed on the sample materials of each testnumber after the aforementioned tempering.

[Tensile Test]

A tensile test was performed in accordance with ASTM E8 (2013). Roundbar tensile test specimens having a diameter of 6.35 mm and a parallelportion length of 35 mm were taken from the center portion of thethickness of the sample materials of each test number. The axialdirection of each of the round bar test specimens was parallel to therolling direction of the sample material (the axial direction of theseamless steel pipe). A tensile test was performed in the atmosphere atnormal temperature (25° C.) using each test number of round bar testspecimens, and the yield strength (MPa) and tensile strength (MPa) wereobtained. Note that, in the present example, the stress at the time of0.7% elongation obtained in the tensile test defined as the yieldstrength for each test number. Further, the largest stress duringuniform elongation was taken as the tensile strength. A ratio of theyield strength to the tensile strength (YS/TS) was adopted as the yieldratio (%). The determined yield strength (YS), tensile strength (TS) andyield ratio (YR) are shown in Table 5.

[Microstructure Determination Test]

With respect to the microstructures of the sample materials of each testnumber, when the yield strength was in a range of 758 to less than 862MPa (110 ksi grade) and the yield ratio was 85% or more, it wasdetermined that the volume ratios of tempered martensite and temperedbainite was 90% or more. In the microstructure of the sample materialsof all of test numbers, the volume ratios of tempered martensite andtempered bainite was 90% or more.

[Amount of Dissolved C Measurement Test]

With respect to the sample materials of each test number, the amount ofdissolved C (mass %) was measured and calculated by the measurementmethod described above. Note that, the TEM used was JEM-2010manufactured by JEOL Ltd., the acceleration voltage was set to 200 kV.For the EDS point analysis the irradiation current was 2.56 nA, andmeasurement was performed for 60 seconds at each point. The observationregions for the TEM observation were 8 μm×8 μm, and observation wasperformed with respect to an arbitrary 10 visual fields. The residualamounts of each element and the concentrations of each element incementite that were used to calculate the amount of dissolved C were aslisted in Table 6.

TABLE 6 Residual Amount Concentration In Cementite Dissolved C Test(mass %) (mass %) Amount Number Steel Fe Cr Mn Mo V Nb Fe Cr Mn Mo (mass%) 2-1 2-A 2.6 0.48 0.09 0.22 — 0.025 81.3 10.9 2.5 5.2 0.024 2-2 2-A2.6 0.48 0.10 0.20 — 0.026 81.0 10.9 2.5 5.6 0.027 2-3 2-A 2.3 0.52 0.100.26 — 0.022 83.3 8.6 2.6 5.5 0.037 2-4 2-B 2.2 0.46 0.10 0.22 0.0660.023 81.5 10.4 2.5 5.7 0.037 2-5 2-B 2.2 0.43 0.13 0.27 0.063 0.02484.0 8.6 2.7 4.6 0.029 2-6 2-C 2.4 0.24 0.11 0.25 0.068 0.026 85.3 8.12.1 4.5 0.032 2-7 2-C 2.3 0.21 0.10 0.22 0.071 0.017 85.0 7.3 2.6 5.10.047 2-8 2-D 2.2 0.32 0.11 0.20 0.071 0.023 82.9 9.7 2.3 5.0 0.047 2-92-D 2.1 0.32 0.11 0.25 0.068 0.025 82.0 9:4 2.5 6.1 0.050 2-10 2-E 2.40.47 0.11 0.21 0.071 0.022 83.1 8.6 3.0 5.3 0.022 2-11 2-F 2.4 0.45 0.120.21 0.070 0.021 85.2 7.6 2.5 4.7 0.031 2-12 2-G 2.4 0.45 0.15 0.190.071 0.025 84.0 7.7 2.8 5.5 0.033 2-13 2-H 2.4 0.47 0.11 0.12 0.0710.021 84.8 7.7 2.7 4.8 0.012 2-14 2-I 2.2 0.47 0.11 0.22 0.063 0.02884.8 7.1 2.9 5.1 0.025 2-15 2-J 2.3 0.45 0.11 0.23 0.072 0.021 84.3 7.32.9 5.5 0.048 2-16 2-K 2.4 0.46 0.11 0.27 0.072 0.021 84.4 7.2 2.9 5.50.016 2-17 2-L 2.5 0.44 0.11 0.23 0.063 0.021 83.9 7.7 2.9 5.5 0.0172-18 2-M 2.5 0.45 0.11 0.22 0.071 0.021 84.3 7.4 2.9 5.5 0.016 2-19 2-N2.1 0.45 0.11 0.35 0.065 0.021 84.0 7.5 2.9 5.5 0.028 2-20 2-A 2.7 0.510.11 0.26 — 0.023 85.9 7.2 2.9 4.0 0.005 2-21 2-A 2.7 0.52 0.11 0.26 —0.024 83.4 9.7 2.8 4.0 0.004 2-22 2-A 2.7 0.54 0.11 0.26 — 0.028 78.314.8 2.8 4.1 0.003 2-23 2-B 2.5 0.55 0.11 0.23 0.061 0.024 83.5 9.7 2.44.4 0.006 2-24 2-B 2.4 0.56 0.11 0.26 0.077 0.023 85.6 7.3 2.9 4.2 0.0042-25 2-C 2.6 0.38 0.10 0.26 0.070 0.025 83.5 9.6 3.0 3.9 0.005 2-26 2-C2.6 0.38 0.11 0.28 0.069 0.028 85.5 7.1 2.9 4.5 0.004 2-27 2-D 2.5 0.550.11 0.23 0.061 0.024 83.3 10.2 2.4 4.0 0.005 2-28 2-D 2.4 0.56 0.110.26 0.077 0.023 85.4 7.2 2.9 4.5 0.005 2-29 2-A 2.1 0.46 0.10 0.19 —0.025 80.3 10.1 3.3 6.3 0.065 2-30 2-O 2.3 0.47 0.12 0.29 — 0.021 81.99.6 2.6 5.9 0.036

[Dislocation Density Measurement Test]

Test specimens for use for dislocation density measurement by theaforementioned method were taken from the sample material of each testnumber. In addition, the dislocation density (m⁻²) was determined by theaforementioned method. The determined dislocation density (×10¹⁴×m⁻²) isshown in Table 5.

[DCB Test]

With respect to the sample materials of each test number, a DCB test wasconducted in accordance with “Method D” of NACE TM0177-2005, and the SSCresistance was evaluated. Specifically, the DCB test was performed in asimilar manner to Example 1 except that the thickness t of the wedgedescribed in FIG. 2B was made 2.92 (mm). The arithmetic average of thethree fracture toughness value K_(ISSC) (MPa√m) was determined, anddefined as the fracture toughness value K_(ISSC) (MPa√m) of the samplematerial of the relevant test number.

For the sample material of each test number, the obtained fracturetoughness values K_(ISSC) are shown in Table 5. If the fracturetoughness value K_(ISSC) that was defined as described above was 27.5MPa√m or more, it was determined that the SSC resistance was good. Notethat, the clearance between the arms when the wedge is driven in priorto immersion in the test bath influences the K_(ISSC) value.Accordingly, actual measurement of the clearance between the arms wasperformed in advance using a micrometer, and it was also confined thatthe clearance was within the range in the API standards.

[Test Results]

The test results are shown in Table 5.

Referring to Table 4 and Table 5, the chemical compositions of thesample materials of Test Numbers 2-1 to 2-19 were appropriate, the yieldstrength was in the range of 758 to less than 862 MPa (110 ksi grade),and the yield ratio was 85% or more. In addition, the amount ofdissolved C was in the range of 0.010 to 0.050 mass %. As a result, theK_(ISSC) value was 27.5 MPa√m or more and excellent SSC resistance wasexhibited. Note that, the dislocation density of the sample materials ofTest Numbers 2-1 to 2-19 were within the range of 4.4×10¹⁴ to less than6.5×10¹⁴ m⁻².

On the other hand, for the sample materials of Test Numbers 2-20 to2-28, the cooling rate after tempering (CR₆₀₀₋₂₀₀) was too slow.Consequently, the amount of dissolved C was less than 0.010 mass %. As aresult, the fracture toughness value K_(ISSC) was less than 27.5 MPa√mand excellent SSC resistance was not exhibited. Note that, thedislocation density of the sample materials of Test Numbers 2-20 to 2-28were within the range of 4.4×10¹⁴ to less than 6.5×10¹⁴ m⁻².

For the sample material of Test Number 2-29, the cooling rate aftertempering (CR₆₀₀₋₂₀₀) was too fast. Consequently, the amount ofdissolved C was more than 0.050 masse. As a result, the fracturetoughness value K_(ISSC) was less than 27.5 MPa√m and excellent SSCresistance was not exhibited. Note that, the dislocation density of thesample material of Test Number 2-29 was within the range of 4.4×10¹⁴ toless than 6.5×10¹⁴ m⁻².

For the sample material of Test Number 2-30, the Mo content was too low.As a result, the fracture toughness value K_(ISSC) was less than 27.5MPa√m and excellent SSC resistance was not exhibited. Note that, thedislocation density of the sample material of Test Number 2-30 waswithin the range of 4.4×10¹⁴ to less than 6.5×10¹⁴ m⁻².

An embodiment of the present invention has been described above.However, the embodiment described above is merely an example forimplementing the present invention. Accordingly, the present inventionis not limited to the above embodiment, and the above embodiment can beappropriately modified and performed within a range that does notdeviate from the gist of the present invention.

INDUSTRIAL APPLICABILITY

The steel material according to the present invention is widelyapplicable to steel materials to be utilized in a sour environment, andpreferably can be utilized as a steel material for oil wells that isutilized in an oil well environment, and further preferably can beutilized as oil-well steel pipes, such as casing, tubing and line pipes.

The invention claimed is:
 1. A steel material having a chemicalcomposition consisting of, in mass %, C: 0.20 to 0.50%, Si: 0.05 to0.50%, Mn: 0.05 to 1.00%, P: 0.030% or less, S: 0.0100% or less, Al:0.005 to 0.100%, Cr: 0.10 to 1.50%, Mo: 0.25 to 1.50%, Ti: 0.002 to0.050%, N: 0.0100% or less, O: 0.0100% or less, B: 0 to 0.0050%, V: 0 to0.30%, Nb: 0 to 0.100%, Ca: 0 to 0.0100%, Mg: 0 to 0.0100%, Zr: 0 to0.0100%, rare earth metal: 0 to 0.0100%, Co: 0 to 0.45%, W: 0 to 0.50%,Ni: 0 to 0.50%, Cu: 0 to 0.50%, and with the balance being Fe andimpurities, an amount of dissolved C within a range of 0.010 to 0.050mass %, a yield strength within a range of 655 to less than 862 MPa, anda yield ratio of 85% or more.
 2. The steel material according to claim1, wherein the chemical composition contains: B: 0.0001 to 0.0050%. 3.The steel material according to claim 1, wherein the chemicalcomposition contains one or more types of elements selected from thegroup consisting of: V: 0.01 to 0.30%, and Nb: 0.002 to 0.100%.
 4. Thesteel material according to claim 1, wherein the chemical compositioncontains one or more types of elements selected from the groupconsisting of: Ca: 0.0001 to 0.0100%, Mg: 0.0001 to 0.0100%, Zr: 0.0001to 0.0100%, and rare earth metal: 0.0001 to 0.0100%.
 5. The steelmaterial according to claim 1, wherein the chemical composition containsone or more types of elements selected from the group consisting of: Co:0.02 to 0.45%, and W: 0.02 to 0.50%.
 6. The steel material according toclaim 1, wherein the chemical composition contains one or more types ofelements selected from the group consisting of: Ni: 0.02 to 0.50%, andCu: 0.02 to 0.50%.
 7. The steel material according to claim 1, whereinthe yield strength is within the range of 655 to less than 758 MPa. 8.The steel material according to claim 1, wherein the yield strength iswithin the range of 758 to less than 862 MPa.
 9. The steel materialaccording to claim 1, wherein the steel material is an oil-well steelpipe.
 10. The steel material according to claim 1, wherein the steelmaterial is a seamless steel pipe.